[Feature Request]: TubeArchivist integration for subscriptions / downloading

### Guidelines

- [X] I have searched the issue tracker for [open](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aopen+is%3Aissue) and [closed](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aissue+is%3Aclosed) issues that are similar to the feature request I want to file, without success.
- [X] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.
- [X] This issue contains only one feature request.

### Problem Description

As someone who uses FreeTube for general YouTube browsing, and TubeArchivist (https://github.com/tubearchivist/tubearchivist) for archiving videos I want to watch later / back up etc, it would be great to have an integration between the two.

### Proposed Solution

How this could work, if integration was enabled in FreeTube:

- At a basic level, having a download button on videos which sent the download instruction to TubeArchivist would be great
- Subscriptions could perhaps also be optionally be synced between the two (so when subscribing on FreeTube this synced the subscription on TubeArchivist also)

### Alternatives Considered

At present alternative is to copy the video or channel URL from FreeTube, and manually paste it into TubeArchivist web interface to trigger download or subscribe.

### Issue Labels

new feature, new optional setting, support for external software

### Additional Information

_No response_