feat: promote IdToken and JWT features

[chingor13]

These features have been available for months and have not received bug reports.

[codecov]

Codecov Report

Merging #538 (9e7f1ee) into master (9a11c50) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #538   +/-   ##
=========================================
  Coverage     79.92%   79.92%           
  Complexity      421      421           
=========================================
  Files            28       28           
  Lines          1978     1978           
  Branches        215      215           
=========================================
  Hits           1581     1581           
  Misses          284      284           
  Partials        113      113           

Impacted Files	Coverage Δ	Complexity Δ
...m/google/auth/oauth2/ComputeEngineCredentials.java	83.89% <ø> (ø)	35.00 <0.00> (ø)
...uth2_http/java/com/google/auth/oauth2/IdToken.java	82.60% <ø> (ø)	10.00 <0.00> (ø)
...ava/com/google/auth/oauth2/IdTokenCredentials.java	86.66% <ø> (ø)	8.00 <0.00> (ø)
...p/java/com/google/auth/oauth2/IdTokenProvider.java	87.50% <ø> (ø)	0.00 <0.00> (ø)
...om/google/auth/oauth2/ImpersonatedCredentials.java	86.40% <ø> (ø)	16.00 <0.00> (ø)
.../google/auth/oauth2/ServiceAccountCredentials.java	82.94% <ø> (ø)	52.00 <0.00> (ø)
...ttp/java/com/google/auth/oauth2/TokenVerifier.java	83.60% <ø> (ø)	11.00 <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 9a11c50...9e7f1ee. Read the comment docs.