Allow to override EndpointsMethodHandler creation in EndpointsServlet

[clementdenis]

This very small change would allow to customize serialization mechanisms and other Endpoints internals that are currently hidden.

[codecov-io]

Codecov Report

Merging #160 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #160      +/-   ##
============================================
+ Coverage        80%   80.02%   +0.02%     
- Complexity     1680     1682       +2     
============================================
  Files           156      156              
  Lines          5597     5598       +1     
  Branches        731      731              
============================================
+ Hits           4478     4480       +2     
  Misses          839      839              
+ Partials        280      279       -1

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/api/server/spi/EndpointsServlet.java	89.83% <100%> (+0.17%)	18 <1> (+1)	⬆️
.../server/spi/discovery/CommonPathPrefixBuilder.java	100% <0%> (+5.55%)	9% <0%> (+1%)	⬆️

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