Allow the Configuration of Additional Headers on the Jaeger Query HTTP API

[joe-elliott]

Which problem is this PR solving?

• Fixes Add CORS handling to query API  #2039

Short description of the changes

• Added a parameter query.additional-headers that allows for multiple headers to be added to jaeger query's api
• Configurable from the CLI
  • --query.additional-headers "Access-Control-Allow-Origin: https://mozilla.org" --query-additional-headers "Access-Control-Expose-Headers: X-My-Custom-Header"
• Configurable from yaml

  query:
    additional-headers:
      - "Access-Control-Allow-Origin: https://mozilla.org"
      - "Access-Control-Expose-Headers: X-My-Custom-Header"
  

• Added a config.StringSlice object that can be used to pass string slices through pflag/viper

[joe-elliott]

Functionality confirmed with local testing. Dropping Draft status.

[codecov]

Codecov Report

Merging #2056 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2056      +/-   ##
=========================================
+ Coverage   97.39%   97.4%   +<.01%     
=========================================
  Files         207     209       +2     
  Lines       10307   10340      +33     
=========================================
+ Hits        10039   10072      +33     
  Misses        224     224              
  Partials       44      44

Impacted Files	Coverage Δ
cmd/query/app/additional_headers_handler.go	100% <100%> (ø)
cmd/query/app/flags.go	100% <100%> (ø)	⬆️
cmd/query/app/server.go	91.78% <100%> (+0.11%)	⬆️
pkg/config/string_slice.go	100% <100%> (ø)

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[yurishkuro]

Thanks!

[yurishkuro]

restarting unit tests to get the coverage numbers (call to codecov timed out)