Decode error due to invalid typ header

[AdnaneOnFire]

I noticed the following behavior while trying to validate third party produced tokens. It seems that the only allowed value for the typ header is jwt. This is due to the following https://github.com/authlib/joserfc/blob/main/src/joserfc/jwt.py#L109

I feel this is contradictory with the assessment made for the following older issue , which allowed a more free form typ header. At any rate if we can encode a jwt with typ other than jwt , we should be able to decode the generated jwt.

Can you maybe validate typ header as a string with a reasonable max length instead ? or maybe accept any string with "mime type format" as valid. This make sens as , according to https://www.rfc-editor.org/rfc/rfc7519#section-5.1 :

The "typ" (type) Header Parameter defined by [[JWS](https://www.rfc-editor.org/rfc/rfc7519#ref-JWS)] 
and [[JWE](https://www.rfc-editor.org/rfc/rfc7519#ref-JWE)] is used by JWT applications to declare 
the media type [[IANA.MediaTypes](https://www.rfc-editor.org/rfc/rfc7519#ref-IANA.MediaTypes)] 
of this complete JWT.

I would be happy to make a PR in this direction.

[lepture]

Thanks, a PR is welcome.