Expose EndpointsServlet internals to allow customizing JSON serialization

[clementdenis]

The goal of these changes is to allow customization of the JSON serialization process of API responses, by subclassing EndpointsServlet, EndpointsMethodHandler and ServletResponseResultWriter.

An example of such a customization is available in this project.
PartialResponseEndpointsServlet provides a (hopefully) compatible implementation of the partial response feature that was dropped in Endpoints v2 (see #75).
It has some with additional goodies such as eager "fields" validation (Google APIs seem to validate it after executing backend methods), and JSON Pointer support.

[codecov-io]

Codecov Report

Merging #166 into master will decrease coverage by 0.04%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master     #166      +/-   ##
============================================
- Coverage     80.12%   80.07%   -0.05%     
- Complexity     1689     1692       +3     
============================================
  Files           157      157              
  Lines          5629     5637       +8     
  Branches        735      735              
============================================
+ Hits           4510     4514       +4     
- Misses          838      843       +5     
+ Partials        281      280       -1

Impacted Files	Coverage Δ	Complexity Δ
...rver/spi/response/ServletResponseResultWriter.java	94.8% <100%> (+0.06%)	18 <1> (+1)	⬆️
...va/com/google/api/server/spi/EndpointsServlet.java	86.88% <50%> (-2.95%)	18 <1> (ø)
...pi/server/spi/handlers/EndpointsMethodHandler.java	92.85% <88.88%> (-7.15%)	11 <3> (+1)
.../server/spi/discovery/CommonPathPrefixBuilder.java	100% <0%> (+5.55%)	9% <0%> (+1%)	⬆️

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