feat(new auth api): add EmbyConnect authentication API

[pXius]

Description

This PR is to serve as an RFC to add EmbyConnect authentication to Jellyseerr.

If follows the exact flow used when login in via https://emby.media, but creates a JellyfinLoginResponse as the last step using the acquired data.

graph TD
    A{Start flow IF username <br> is an email address <br> && <br> Jellyfin auth failed} --> B[Authenticate towards EmbyConnect <br> with email and password]
    B --> C[Get a list of Emby servers linked to user]
    C --> D{Compare if a linked <br> server matches the Emby <br> instance in Jellyseerr config}
    D -->|No Match| F[Throw No matching server Exception]
    D -->|Match Found| E[Do an authentication exchange with the hosted <br> Emby server to get a local userId and authToken]
    E --> G[Construct a JellyfinLoginResponse object <br> and return to existing Jellyfin auth login flow]

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Changes to Code

• Created an emby connect api with the required methods to orchestrate a set of calls towards EmbyConnect to build a JellyseerrLoginResponse object
• Add an EmbyConnect auth call to the Jellyfin login() method as the last auth attempt IF the username is an email address
• Update the post() method on the ExternalApi class to handle request bodies that need to be x-www-form-urlencoded

Notes

• This flow will NOT work as first time setup login as there is no existing configured Emby server to match to. First auth has to use a local account as it is today.
• I attempted to encapsulate as much of the logic in one place to avoid any possible conflicts with feat: Jellyfin/Emby server type setup #685 and other future in-progress work.
• JS is not my strong suit; any comments for improvement are welcome.

Additional Improvements?

• Check new server type from feat: Jellyfin/Emby server type setup #685 before allowing flow. This would avoid calls to EmbyConnect if someone has Jellyfin users with email addresses as names.

To-Dos

• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required)

Issues Fixed or Closed

• Fixes Add authentication with Emby Connect #749

[pXius]

@gauthier-th

Thanks for the feedback.
I've resolved all comments as requested, please confirm that this solution is acceptable.

[pXius]

@gauthier-th
Thanks again for the guidance.

I have resolved the final comment by reverting the baseUrl override in favour of a vanilla fetch() call as requested.

Please let me know if any other changes are required.

[pXius]

Hey @gauthier-th
Anything outstanding to get this merged?

[gauthier-th]

Hey @gauthier-th Anything outstanding to get this merged?

This will be merged, in time.
We also need more people reviewing and testing this, as I don't use EmbyConnect.

[gauthier-th]

LGTM.
This still needs to be tested, though.