feat(gdch): support EC private keys

[diegomarquezp]

Context: b/488439640

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.

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.

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%)

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