add regex match and substitute support to the header mutation

[wbpcode]

Title: One line description

Description:

Now, the header mutation (both the header mutation filter and headers_to_add) could set/add headers by the substitution formatter. (like %REQ(xxxxx)%. But it's impossible to extract more fine-grained part of one header or path. It would be great to
extract sub-value from a header or path by the regex and set the sub-value to a new header.

For example:

        header:
          key: "proxy-target"
          value: "%REQ(:PATH)%"
        rewrite:
           pattern:
             regex: "/prefix/proxy/(.+)"
           substitution: "\\1"
        append_action: APPEND_IF_EXISTS_OR_ADD

[optional Relevant Links:]

Any extra documentation required to understand the issue.

[wbpcode]

https://docs.google.com/document/d/1amCGuj-8KaffTSPzqOnCRkX_s_kwnmORGW83jFmq314/edit?usp=sharing