add regex match and substitute support to the header mutation

*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

```

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>Any extra documentation required to understand the issue.