Add `getHeaders` method to `JsonWebToken`

[sberyozkin]

Currently, JsonWebToken lets users check the claims only, but the token headers, which can provide a lot of useful information, such as the algorithms, as well as custom headers, can not be accessed.

It can be fixed easily with adding jakarta.json.JsonObject getHeaders() method

[sberyozkin]

It is possible to get from the raw_token claim