fix(gdch): support EC private keys

[diegomarquezp]

Context: b/488439640

Implementation originally proposed in b/431924643#comment9

The primary objective is to enable support for Elliptic Curve (EC) keys and non-URI audience formats, aligning the Java SDK with the behavior of the Python and Go implementations. Additionally, the GDCH key creation tool creates EC keys only, meaning the GDCH implementation was not following the convention.

Key Changes

• Algorithm Support: Updated the credential signing process to use the ES256 (ECDSA) algorithm, which is the required standard for GDCH service accounts.
• Audience Flexibility: Changed the apiAudience field from a URI to a String to accommodate "magic" non-URI strings (e.g., specific administrative audiences) required by certain GDCH services.
• OAuth2Utils Refactoring: Enhanced privateKeyFromPkcs8 to accept an algorithm parameter, allowing the library to parse EC keys instead of defaulting exclusively to RSA.
• Test fixes: Fixes in a couple of test files as per Sonarqube analysis.

Testing

• Updated GdchCredentialsTest to include test cases for EC key parsing and token signing.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud