Ability to rename request headers

Now that we can use request headers in the request_headers_to_add option, one might want to rename a header:

    request_headers_to_remove:
    - x-header
    request_headers_to_add:
    - header:
        key: x-new-name
        value: "%REQ(x-header)%"

However this will not work, because HeaderParser::evaluateHeaders removes headers first.

void HeaderParser::evaluateHeaders(Http::HeaderMap& headers,
                                   const StreamInfo::StreamInfo& stream_info) const {
  // Removing headers in the headers_to_remove_ list first makes
  // remove-before-add the default behavior as expected by users.

This was introduced in #5052 : "reverses the order of header evaluation as a solution to deduplicating request/response headers".

There are multiple ways to approach this header-renaming issue.

One is to add request_headers_to_post_remove which will be used after request_headers_to_add.

Another would be request_headers_to_pre_add but "post-remove" is better because headers-to-remove is just a list of names, when headers-to-add is much bulkier.

The most trivial option is to introduce a map<string, string> rename_headers.

There is also an option to add a preserve_request_headers list, to store original request headers somewhere in StreamInfo, and add a new formatter, e.g. %PREQ(header-name)% to access them. It might be useful because such headers won't be removed or modified by filters. However for now I just want to rename a couple of headers and this would be an overkill.

So, how about a new post-remove header list?