[Plugins] [Auth] [JWT] Add addtional JWT headers input

[Moshyfawn]

This PR adds an input field to the JWT Auth plugin for additional JWT headers, enabling support for APIs like App Store Connect that require custom header fields.

Potential improvements for consideration:

• Pre-populate the alg field with the selected algorithm
• Auto-fill iat and exp headers with current timestamp and configurable token lifetime

Happy to discuss and update this PR further!

[gschier]

Great improvement! I dislike having a raw JSON field for headers but that's the best option provided the current plugin APIs.

I'm going to extend the APIs to add a key/value input type so that headers it can be used here, before merging.