feat: support variable when rewrite header in proxy rewrite plugin

[monkeyDluffy6017]

Description

Support for referencing the match result of the regex_uri when rewrite header in the proxy-rewrite plugin, and the match result can be referenced as a variable by other plugins too.

Nginx:

location ~* ^/aaa/(.*)$ {
    ...
    proxy_set_header HDR_TEST $1;
}

Apisix:

"plugins": {
    "proxy-rewrite": {
        "regex_uri": ["^/test/(.*)/(.*)/(.*)", "/$1_$2_$3"],
        "headers": {
            "X-Api:Version": "v2-$1"
        }
    }
},

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[spacewander]

Suggested change

	-- escape is not supported very well, like there si a redundant '\' after escape "$1"
	-- escape is not supported very well, like there is a redundant '\' after escape "$1"

[monkeyDluffy6017]

Done

[spacewander]

Do we need to return a 503 for this error?

[monkeyDluffy6017]

I prefer 500, it's an unexpected condition

[spacewander]

We can merge the two conditions above?

[monkeyDluffy6017]

how? use the re_match first, then use the match result to replace?

[spacewander]

See #9112 (comment)

[monkeyDluffy6017]

Done