panic for unsupported IndexExpr

[johakoch]

Describe the bug
I want to use an environment variable to configure the name of the verifier cookie:

beta_oidc "oidc" {
  ...
  verifier_value = request.cookies[env.VERIFIER_COOKIE_NAME]
}

However, this results in a panic in eval.value.go (clone()) line 241:

	default:
		panic("eval: unsupported expression: " + reflect.TypeOf(exp).String())

The unsupported expression is of type *hclsyntax.IndexExpr.

BTW,

beta_oidc "oidc" {
  ...
  verifier_value = request.cookies["foo"]
}

works.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? 1.5

2. Provide your configuration file *.hcl. Remove sensitive data.

see above

3. Provide a curl call for reproduction
   3.1 You may use your browsers developer console and hit copy as curl.

4. See an error response or error log.

2021/11/10 13:23:05 http: panic serving 127.0.0.1:54298: invalid WriteHeader code 0
goroutine 36 [running]:
net/http.(*conn).serve.func1()
        /usr/local/go/src/net/http/server.go:1801 +0xb9
panic({0xbd1d80, 0xc000646460})
        /usr/local/go/src/runtime/panic.go:1047 +0x266
net/http.checkWriteHeaderCode(...)
        /usr/local/go/src/net/http/server.go:1089
net/http.(*response).WriteHeader(0xc0003c40e0, 0x0)
        /usr/local/go/src/net/http/server.go:1123 +0x5c8
github.com/avenga/couper/server/writer.(*Gzip).writeHeader(0xc0002de800)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/writer/gzip.go:138 +0x22d
github.com/avenga/couper/server/writer.(*Gzip).Close(0xc0002de800)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/writer/gzip.go:96 +0x4e
github.com/avenga/couper/handler/middleware.NewRecordHandler.func1.1.1()
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/middleware/status.go:24 +0x6e
panic({0xbd1d80, 0xc000646450})
        /usr/local/go/src/runtime/panic.go:1038 +0x215
github.com/avenga/couper/eval.clone({0xe3ce90, 0xc0000c6c40})
        /c/Users/johannes.koch.SEVENVAL/src/couper/eval/value.go:241 +0xdb9
github.com/avenga/couper/eval.Value(0xc247e0, {0xe3ce90, 0xc0000c6c40})
        /c/Users/johannes.koch.SEVENVAL/src/couper/eval/value.go:51 +0x77
github.com/avenga/couper/eval.ValueFromAttribute(0xc0002793f0, 0xc00063c840, {0xd0e559, 0xe3ccd0})
        /c/Users/johannes.koch.SEVENVAL/src/couper/eval/value.go:38 +0x46
github.com/avenga/couper/eval.ValueFromBodyAttribute(0xc000223310, {0xe3cdb0, 0xc0002793f0}, {0xd0e559, 0xe})
        /c/Users/johannes.koch.SEVENVAL/src/couper/eval/value.go:29 +0xe8
github.com/avenga/couper/oauth2.AbstractAcClient.GetTokenResponse({{0xe36be8, 0xc0002864c0}, {{0xe25960, 0xc0000468e0}, {0xe34688, 0xc000223310}, {0x7f81b4aeaf58, 0xc000223310}}, {0xc000043d98, 0x4}}, ...)
        /c/Users/johannes.koch.SEVENVAL/src/couper/oauth2/ac.go:67 +0x507
github.com/avenga/couper/accesscontrol.(*OAuth2Callback).Validate(0xc00050bcd0, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/accesscontrol/oauth2.go:32 +0xc8
github.com/avenga/couper/accesscontrol.ListItem.Validate({{0xe25680, 0xc00050bcd0}, {0xe257e0, 0xc00050beb0}, {0xc0002326c0, 0xe}, {0xc000127598, 0x4}}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/accesscontrol/ac.go:43 +0x8a
github.com/avenga/couper/handler.(*AccessControl).ServeHTTP(0xc000132270, {0xe36c48, 0xc0004a4120}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/access_control.go:30 +0x131
github.com/avenga/couper/server.(*HTTPServer).ServeHTTP(0xc000218780, {0xe36c48, 0xc0004a4120}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/http.go:242 +0x9fb
github.com/avenga/couper/handler/middleware.(*TraceHandler).ServeHTTP(0xc00050bf80, {0xe36c48, 0xc0004a4120}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/middleware/trace.go:48 +0x7c2
github.com/avenga/couper/handler/middleware.(*UID).ServeHTTP(0xc000133080, {0xe36c48, 0xc0004a4120}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/middleware/uid.go:56 +0x4cc
github.com/avenga/couper/logging.(*AccessLog).ServeHTTP(0xc000235b00, {0xe36c48, 0xc0004a4120}, 0xc00021c700, {0xe258e0, 0xc000133080})
        /c/Users/johannes.koch.SEVENVAL/src/couper/logging/access_log.go:42 +0x62
github.com/avenga/couper/server.New.func1({0xe36c48, 0xc0004a4120}, 0x1372201)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/http.go:100 +0x3b
net/http.HandlerFunc.ServeHTTP(0xe3cbb8, {0xe36c48, 0xc0004a4120}, 0x1372240)
        /usr/local/go/src/net/http/server.go:2046 +0x2f
github.com/avenga/couper/handler/middleware.NewRecordHandler.func1.1({0xe37728, 0xc0003c40e0}, 0xc00021c700)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/middleware/status.go:30 +0x373
net/http.HandlerFunc.ServeHTTP(0xe3cb10, {0xe37728, 0xc0003c40e0}, 0x1372268)
        /usr/local/go/src/net/http/server.go:2046 +0x2f
github.com/avenga/couper/server.New.func2({0xe37728, 0xc0003c40e0}, 0xc00021c600)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/http.go:104 +0x2b6
net/http.HandlerFunc.ServeHTTP(0xc0000e40fb, {0xe37728, 0xc0003c40e0}, 0x4642ee)
        /usr/local/go/src/net/http/server.go:2046 +0x2f
net/http.serverHandler.ServeHTTP({0xc000554180}, {0xe37728, 0xc0003c40e0}, 0xc00021c600)
        /usr/local/go/src/net/http/server.go:2878 +0x43b
net/http.(*conn).serve(0xc000522140, {0xe3cbb8, 0xc0001680c0})
        /usr/local/go/src/net/http/server.go:1929 +0xb08
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:3033 +0x4e8
...

Expected behavior
I expect the index with an env variable expression to work.