feat: support var in upstream_uri on proxy-rewrite plugin

[BFergerson]

What this PR does / why we need it:

Adds the ability to use ctx.var in upstream_uri on the proxy-rewrite plugin.
Fixes #3133

[Firstsawyou]

Need to add semantic "PR" title, for example: feat: ctx.var in upstream_uri on proxy-rewrite plugin

[Firstsawyou]

We need test cases to cover this feature.

[spacewander]

Please merge master to make CI pass.

[spacewander]

You can write test like

apisix/t/plugin/proxy-rewrite.t

Lines 477 to 526 in 45e909a

	=== TEST 16: set route(rewrite uri args)
	--- config
	location /t {
	content_by_lua_block {
	local t = require("lib.test_admin").test
	local code, body = t('/apisix/admin/routes/1',
	ngx.HTTP_PUT,
	[[{
	"plugins": {
	"proxy-rewrite": {
	"uri": "/plugin_proxy_rewrite_args"
	}
	},
	"upstream": {
	"nodes": {
	"127.0.0.1:1980": 1
	},
	"type": "roundrobin"
	},
	"uri": "/hello"
	}]]
	)
	if code >= 300 then
	ngx.status = code
	end
	ngx.say(body)
	}
	}
	--- request
	GET /t
	--- response_body
	passed
	--- no_error_log
	[error]
	=== TEST 17: rewrite uri args
	--- request
	GET /hello?q=apisix&a=iresty HTTP/1.1
	--- response_body_like eval
	qr/uri: \/plugin_proxy_rewrite_args(
	q: apisix
	a: iresty|
	a: iresty
	q: apisix)
	/
	--- no_error_log
	[error]

[moonming]

ping @membphis