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)

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