Add 'StringNotInSlice' validation function

[ewbankkit]

Add StringNotInSlice(), a validator to ensure that the provided value does NOT match the value of any element in the specified string slice.

$ make test
==> Checking that code complies with gofmt requirements...
go generate ./...
go test ./...
?   	github.com/hashicorp/terraform-plugin-sdk/acctest	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/acctest	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/customdiff	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/helper/encryption	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/hashcode	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/helper/logging	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/mutexkv	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/pathorcontents	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/resource	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/schema	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/structure	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/helper/validation	0.011s
ok  	github.com/hashicorp/terraform-plugin-sdk/httpclient	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/addrs	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/command/format	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/configs	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/configs/configload	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/configs/configschema	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/configs/hcl2shim	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/dag	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/earlyconfig	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/flatmap	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/helper/config	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/helper/didyoumean	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/helper/experiment	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/httpclient	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/initwd	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/lang	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/lang/blocktoattr	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/lang/funcs	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/modsdir	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/moduledeps	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/plans	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/plans/internal/planproto	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/plans/objchange	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/plans/planfile	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/plugin/convert	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/plugin/discovery	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/plugin/mock_proto	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/providers	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/provisioners	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/registry	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/registry/regsrc	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/registry/response	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/registry/test	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/states	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/states/statefile	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/tfdiags	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/vault/helper/pgpkeys	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/vault/sdk/helper/compressutil	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/internal/vault/sdk/helper/jsonutil	(cached)
?   	github.com/hashicorp/terraform-plugin-sdk/internal/version	[no test files]
?   	github.com/hashicorp/terraform-plugin-sdk/meta	[no test files]
ok  	github.com/hashicorp/terraform-plugin-sdk/plugin	(cached)
ok  	github.com/hashicorp/terraform-plugin-sdk/terraform	(cached)

[ghost]

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