Enrich and expose internal methods related to authentication

[clementdenis]

• add missing TokenInfo fields, and expose GoogleAuth.getTokenInfoRemote
• Store validated TokenInfo or GoogleIdToken in request attribute, for custom Authenticators usage
• Expose Auth/PeerAuth.instantiate(Peer)Authenticator to enforce @singleton contract
• Add Javadoc on request Attribute constants

The main goal of these changes is to allow implementing custom Authenticators more easily and efficiently, such as what is implement in this project.
It does not change any of the existing behavior.

[codecov-io]

Codecov Report

Merging #165 into master will decrease coverage by 0.08%.
The diff coverage is 68.57%.

@@             Coverage Diff              @@
##             master     #165      +/-   ##
============================================
- Coverage     80.12%   80.03%   -0.09%     
+ Complexity     1689     1687       -2     
============================================
  Files           157      158       +1     
  Lines          5629     5630       +1     
  Branches        735      734       -1     
============================================
- Hits           4510     4506       -4     
- Misses          838      843       +5     
  Partials        281      281

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/google/api/server/spi/auth/GoogleAuth.java	80.48% <0%> (-5.23%)	33 <0> (ø)
...api/server/spi/auth/GoogleOAuth2Authenticator.java	87.5% <100%> (+0.4%)	10 <0> (ø)	⬇️
...a/com/google/api/server/spi/request/Attribute.java	100% <100%> (ø)	10 <1> (ø)	⬇️
...le/api/server/spi/auth/GoogleJwtAuthenticator.java	85.36% <100%> (+0.36%)	11 <0> (ø)	⬇️
.../server/spi/auth/GoogleAppEngineAuthenticator.java	85.45% <100%> (ø)	16 <0> (ø)	⬇️
...n/java/com/google/api/server/spi/request/Auth.java	75.86% <75%> (-1.64%)	9 <2> (-1)
.../main/java/com/google/api/server/spi/PeerAuth.java	88.88% <75%> (+2.22%)	8 <2> (-1)	⬇️
...va/com/google/api/server/spi/config/Singleton.java	76.47% <76.47%> (ø)	0 <0> (?)

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