feat: add TokenVerifier class that can verify RS256/ES256 tokens

[chingor13]

Exposes interface for validating Google signed RS256/ES256 JWT tokens.

This implementation returns a JsonWebSignature instance (from google-http-client) on success and throws a VerificationException on any error.

Usage:

TokenVerifier tokenVerifier = TokenVerifier.newBuilder()
  .setAudience("audience-to-verify")
  .setIssuer("issuer-to-verify")
  .build();
try {
  JsonWebSignature jsonWebSignature = tokenVerifier.verify(tokenString);
  // optionally verify additional claims
  jsonWebSignature
} catch (TokenVerifier.VerificationException e) {
  // invalid token
}

Fixes #85

[codecov]

Codecov Report

Merging #420 into master will increase coverage by 0.36%.
The diff coverage is 83.87%.

@@             Coverage Diff              @@
##             master     #420      +/-   ##
============================================
+ Coverage     79.50%   79.86%   +0.36%     
- Complexity      397      409      +12     
============================================
  Files            27       28       +1     
  Lines          1800     1927     +127     
  Branches        186      201      +15     
============================================
+ Hits           1431     1539     +108     
- Misses          269      280      +11     
- Partials        100      108       +8     

Impacted Files	Coverage Δ	Complexity Δ
...ttp/java/com/google/auth/oauth2/TokenVerifier.java	83.87% <83.87%> (ø)	12.00 <12.00> (?)
...tp/java/com/google/auth/oauth2/UserAuthorizer.java	77.59% <0.00%> (+0.12%)	20.00% <0.00%> (ø%)
...om/google/auth/appengine/AppEngineCredentials.java	75.47% <0.00%> (+0.96%)	16.00% <0.00%> (ø%)
..._http/java/com/google/auth/oauth2/OAuth2Utils.java	67.21% <0.00%> (+1.63%)	12.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 662bd80...96aabab. Read the comment docs.

[bshaffer]

@BenWhitehead @elharo If you have no additional concerns can one of you approve this PR?