feat(oauth): Support multiple values in the “aud” claim for JWT client assertions

[sji-buttercup]

• Allow the audience field to be a single audience or a comma-separated list of audiences
• Add unit tests for audience containing a single value or multiple values
• Update doc

Manual tests were successful for both:

spark.sql.catalog.iceberg.rest.auth.oauth2.client-assertion.jwt.audience=https://auth1.example.com/token
spark.sql.catalog.iceberg.rest.auth.oauth2.client-assertion.jwt.audience=https://auth1.example.com/token,https://auth2.example.com/token

Now the audience will always be a list, but given that the oauth2-oidc-sdk will always convert one-element to a string when generating the assertion, I believe it should be fine:

		if (audStringList != null) {
			if (audStringList.size() == 1) {
				o.put("aud", audStringList.get(0));
			} else {
				o.put("aud", audStringList);
			}
		}

When I extract the claims set from the assertion with assertion.getJWTClaimsSet().toJSONObject(), I can see that for a single audience, it's still a single string instead of a list.

Fixes #184

[adutra]

Thank you for the quick changes @sji-buttercup !

I left a few minor remarks but this is almost good to go. Thanks!

[adutra]

Thank you for your contribution @sji-buttercup !