Remove dependency on App Engine SDK in EndpointsAuthenticator

[clementdenis]

No description provided.

[RaHery]

@clementdenis Could you check please, some tests did not pass.
Thanks

[clementdenis]

Rebased on master, should fix it.

[clementdenis]

Actually, master won't pass tests until #37 is merged.
Can you review this it? I'll rebase on updated master once merged.

[codecov-io]

Codecov Report

Merging #39 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master      #39      +/-   ##
============================================
- Coverage      81.8%   81.76%   -0.05%     
+ Complexity     1821     1820       -1     
============================================
  Files           159      159              
  Lines          6634     6634              
  Branches        871      872       +1     
============================================
- Hits           5427     5424       -3     
- Misses          883      885       +2     
- Partials        324      325       +1

Impacted Files	Coverage Δ	Complexity Δ
...le/api/server/spi/auth/EndpointsAuthenticator.java	94.11% <0%> (-5.89%)	6 <0> (ø)
.../server/spi/auth/GoogleAppEngineAuthenticator.java	82.14% <0%> (-3.58%)	15% <0%> (-1%)

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