Make list of Access-Control-Allow-Headers configurable.

[jalvz]

The configurable list is appended to Content-Type, Content-Encoding and Accept, which are allways included.

Closes #2497

• Needs changelog
• Needs docs
• Needs cloud whitelist?

[robgil]

Codecov Report

Merging #3299 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3299   +/-   ##
=======================================
  Coverage   79.82%   79.82%           
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  Files         107      107           
  Lines        5625     5627    +2     
=======================================
+ Hits         4490     4492    +2     
  Misses       1135     1135           

[jalvz]

fyi @bmorelli25

[axw]

Default configuration:

$ curl -XOPTIONS -v http://localhost:8200/intake/v2/rum/events
...
Access-Control-Allow-Headers: Content-Type, Content-Encoding, Accept
...

With -E apm-server.rum.allow_headers=[Foo,Bar]:

$ curl -XOPTIONS -v http://localhost:8200/intake/v2/rum/events
...
Access-Control-Allow-Headers: Foo, Bar, Content-Type, Content-Encoding, Accept
...