[7.x] Support custom response headers for RUM endpoints (#3820)

[jalvz]

Backports the following commits to 7.x:

• Support custom response headers for RUM endpoints (Support custom response headers for RUM endpoints #3820)

[codecov-commenter]

Codecov Report

Merging #3825 into 7.x will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              7.x    #3825      +/-   ##
==========================================
- Coverage   79.91%   79.90%   -0.01%     
==========================================
  Files         136      137       +1     
  Lines        6158     6166       +8     
==========================================
+ Hits         4921     4927       +6     
- Misses       1237     1239       +2     

Impacted Files	Coverage Δ
beater/config/rum.go	79.41% <ø> (ø)
beater/api/mux.go	80.32% <100.00%> (+0.16%)	⬆️
beater/middleware/response_headers_middleware.go	100.00% <100.00%> (ø)
beater/jaeger/common.go	78.78% <0.00%> (-6.07%)	⬇️

[axw]

Verified with BC2.

Configured apm-server with -E apm-server.rum.response_headers={key: 'value'}, and confirmed that each of the RUM endpoints responded with an additional header, "Key: value":

• /config/v1/rum/agents
• /intake/v2/rum/events
• /intake/v3/rum/events

Also confirmed that non-RUM endpoints do not respond with the header. This includes the root endpoint.