router: add REQ(header-name) header formatter

[ascheglov]

request_headers_to_add and response_headers_to_add can now use "%REQ(header-name)%"

Use cases:

• rename a request header:

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

• echo a request header, including envoy-generated headers:

    response_headers_to_add:
    - header:
        key: x-header-echo
        value: "%REQ(x-header)%"
    - header:
        key: x-request-id
        value: "%REQ(x-request-id)%"

Risk Level: low
Testing: unit tests
Doc Changes: yes
Release Notes: yes
Fixes #8683

[zuercher]

Thanks! I have some comments, but I think this will be a nice feature to have.

[zuercher]

Also there's an implementation of StreamInfo in test/common/stream_info/test_util.h that needs to be updated.

[zuercher]

I think if you merge master, the coverage test will stop failing. There were a few commits to increase coverage not long after you branched.

[zuercher]

Thanks!

@dio can you have a look as well?

[dio]

Awesome.