Allow to transform `HttpRequest` or `HttpResponse`

[0x1306e6d]

Add mapRequest and mapResponse methods to UpstreamBuilder. They allow users to transform HttpRequest or HttpResponse directly. It would be useful if a user want to customize something that Tollgate doesn't support yet.

Existing disallowRequestHeaders and disallowResponseHeaders now uses mapRequest and mapResponse respectively. Implementation of disallow is extracted from DefaultUpstream to independent Function implementation, DisallowRequestHeadersFunction and DisallowResponseHeadersFunction. Because both functions accept Predicate to test which headers to disallow, we can provide more easier way to disallow headers in future.

[codecov[bot]]

Codecov Report

Merging #140 (d460b49) into master (8e34acd) will increase coverage by 1.15%. The diff coverage is 95.55%.

@@             Coverage Diff              @@
##             master     #140      +/-   ##
============================================
+ Coverage     67.51%   68.66%   +1.15%     
- Complexity      127      137      +10     
============================================
  Files            25       27       +2     
  Lines           434      450      +16     
  Branches         28       23       -5     
============================================
+ Hits            293      309      +16     
  Misses          116      116              
  Partials         25       25              

Impacted Files	Coverage Δ
...llgate/gateway/DisallowRequestHeadersFunction.java	91.66% <91.66%> (ø)
...lgate/gateway/DisallowResponseHeadersFunction.java	91.66% <91.66%> (ø)
...a/dev/gihwan/tollgate/gateway/DefaultUpstream.java	100.00% <100.00%> (ø)
...a/dev/gihwan/tollgate/gateway/UpstreamBuilder.java	61.11% <100.00%> (+11.11%)	:arrow_up:

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