RANGER-5533:Verify JWT Issuer Claims if present

[ChinmayHegde24]

Support for validating the iss (issuer) claim in JWT has been added as part of this PR.

[kumaab]

Instead of adding a standalone validateIssuer() method, consider using Nimbus's DefaultJWTClaimsVerifier, which would consolidate issuer, audience, and expiration checks in one place:

JWTClaimsSet.Builder exactMatchBuilder = new JWTClaimsSet.Builder();
String               issuer            = config.getProperty(KEY_JWT_ISSUER);
if (StringUtils.isNotBlank(issuer)) {
    exactMatchBuilder.issuer(issuer);
}

Set<String> acceptedAudiences = null;
String      audiencesStr      = config.getProperty(KEY_JWT_AUDIENCES);
if (StringUtils.isNotBlank(audiencesStr)) {
    acceptedAudiences = new HashSet<>(Arrays.asList(audiencesStr.split(",")));
}

claimsVerifier = new DefaultJWTClaimsVerifier<>(acceptedAudiences, exactMatchBuilder.build(), null, null);
claimsVerifier.verify(jwtToken.getJWTClaimsSet(), null);

This would replace all three of validateExpiration(), validateAudiences(), and the proposed validateIssuer() with a single, well-tested library call. It also gets clock skew handling for free (the verifier has a configurable skew, defaulting to 60 seconds).