Add claim path delimiter configuration for JwtGrantedAuthoritiesConverter

[BeforeOleg]

Expected Behavior
JwtGrantedAuthoritiesConverter should provide an ability to access authorities as nested claim.
As well as it should be included into OAuth2ResourceServerJwtConfiguration.JwtConverterConfiguration.

private Collection<String> getAuthorities(Jwt jwt) {
    String claimName = getAuthoritiesClaimName(jwt);
    ...
    Object authorities = getClaim(jwt, claimName);
    ...
}


private static Object getClaim(Jwt jwt, String claimName) {
    if (this.authoritiesClaimNamePathDelimiter != null) {
        String[] path = claimName.split(this.authoritiesClaimNamePathDelimiter);
        if (path.length > 1) {
            return getNestedClaim(jwt.getClaims(), path);
        }
    }
    return jwt.getClaim(claimName);
}

private static Object getNestedClaim(Map<String, Object> map, String[] path) {
    Object current = map;
    for (String key : path) {
        if (current instanceof Map) {
            current = map.get(key);
        } else {
            return null;
        }
    }
    return current;
}

Current Behavior
Does not provide ability to access nested claims.

private Collection<String> getAuthorities(Jwt jwt) {
    String claimName = getAuthoritiesClaimName(jwt);
    ...
    Object authorities = jwt.getClaim(claimName);
    ...
}

Context
What are you trying to accomplish?
I want to have an ability to access authorities stored in the custom property:

{
  "realm_access": {
    "roles": [
      "admin",
      "manager",
      "user"
    ]
  }
}

What other alternatives have you considered? Are you aware of any workarounds?
Obviously, I can create a custom converter or store claims in the root level of jwt.
However, I find it useful to have it included into the framework.