Custom User-Agent header and/or other headers for back-channel request

[zandbelt]

Discussed in #1151

^{Originally posted by studersi December 19, 2023}
When debugging issues with mod_auth_openidc and mod_oauth2, we sometimes struggle with request correlation.

It would be helpful to set custom user-agent string for back-channel request to associate them with a particular Apache instance.

Further, it could be helpful to add custom tracing headers like the traceparent header (Trace Context - Traceparent Header to back-channel request for tracing the access across all services that are involved in processing a request.