Add option to disallow request or response headers

[0x1306e6d]

Add disallowRequestHeaders and disallowResponseHeaders methods to UpstreamBuilder. It allows users to make some HTTP headers not to be included in an HTTP request to upstream or response from the upstream. It'd be useful to disallow kind of sensitive headers like authorization header.

Move executing an HTTP request from UpstreamHttpService to Upstream and specific execution is hidden. We can provide more options through UpstreamBuilder.

[codecov[bot]]

Codecov Report

Merging #139 (1f2a346) into master (f116a27) will increase coverage by 2.97%. The diff coverage is 91.66%.

@@             Coverage Diff              @@
##             master     #139      +/-   ##
============================================
+ Coverage     64.54%   67.51%   +2.97%     
- Complexity      114      127      +13     
============================================
  Files            25       25              
  Lines           392      434      +42     
  Branches         19       28       +9     
============================================
+ Hits            253      293      +40     
  Misses          116      116              
- Partials         23       25       +2     

Impacted Files	Coverage Δ
...ain/java/dev/gihwan/tollgate/gateway/Upstream.java	20.00% <ø> (ø)
...a/dev/gihwan/tollgate/gateway/UpstreamBuilder.java	50.00% <86.66%> (+27.77%)	:arrow_up:
...ollgate/hocon/DefaultHoconGatewayConfigurator.java	64.63% <86.95%> (+4.36%)	:arrow_up:
...a/dev/gihwan/tollgate/gateway/DefaultUpstream.java	100.00% <100.00%> (ø)
...v/gihwan/tollgate/gateway/UpstreamHttpService.java	100.00% <100.00%> (ø)

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