feat: Add support for custom headers in Seerr authentication

[xkirbys]

Pull Request Description

Adds support for custom headers in Seerr/Jellyseerr authentication, enabling users to configure additional HTTP headers when connecting to their Seerr instance.

Changes include:

• Added customHeaders field to SeerrCredentialsModel to store custom header configurations
• Updated the Seerr Chopper service to include custom headers in all API requests
• Added a custom headers field to the Seerr connection dialog
• Modified authentication and API provider logic to handle custom headers throughout the request lifecycle

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.

[PartyDonut]

Thanks for looking at implementing this. Code looks good.

Left one small change request.

[PartyDonut]

Please move the string literals to the app_en.arb translation file

[PartyDonut]

That was quick 😄, thanks.