Support custom response headers for RUM endpoints

[jalvz]

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:

• documentation
• logging (add log lines, choose appropriate log selector, etc.)
• metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

Configure some headers in apm-server.rum.response_headers, send a request to any/all rum endpoints, and observe those headers in the response. Should work for both strings and lists

Related issues

Closes #3696
Docs issue #3821

[axw]

Looks good, just some minor suggestions for the YAML comments.

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #3820 updated]

• Start Time: 2020-05-27T12:02:48.252+0000

• Duration: 47 min 0 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3230
Skipped	145
Total	3375

[jalvz]

I guess we need to white-list this on Cloud. Any contrary opinions?

[codecov-commenter]

Codecov Report

Merging #3820 into master will not change coverage.
The diff coverage is n/a.

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##           master    #3820   +/-   ##
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  Coverage   80.01%   80.01%           
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  Files         137      137           
  Lines        6195     6195           
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  Hits         4957     4957           
  Misses       1238     1238