Extending JWT claims validation to support other claims

Based on [source](https://github.com/apache/couchdb/blob/main/src/jwtf/src/jwtf.erl), I understand only limited number of JWT claims can be validated. Trying to validate other claims result in error `unknown_checks`.
I would like to ask for introducing validation any provided claim.

In my case, I use an SSO of a huge Organisation, where many users can define their own applications/clients (signed with same SSO key). Without validating `aud`, anyone could create another application with roles that my CouchDB instance accepts.

## Desired Behaviour

When provided a config like below, the claim `aud` should be verified: both if it exists and if it matched provided `my-application` value.

`required_claims = exp, {aud, "my-application"}`

I believe it's worth allowing such a validation for any custom claim (only to check existence and value matching, if provided).

## Possible Solution

I believe the [source](https://github.com/apache/couchdb/blob/main/src/jwtf/src/jwtf.erl) should not limit the check only to claims specified in line 45. There could be a function providing a "general" claim check, no matter what it is exactly.

Start with src/jwtf/src/jwtf.erl, especially the claim checks referenced at line 45, and review how required_claims is parsed. The change is complete when a configuration such as required_claims = exp, {aud, "my-application"} validates aud for existence and the supplied value, while supporting other provided claims without unknown_checks.

Written by the indexing model from the issue text.