Add more subscriptions export formats

[noraj]

Describe the feature

In addition to the Piped JSON export format, support:

• OPML format
• OPML format (for NewPipe and FreeTube)
• .db (FreeTube)
• JSON (NewPipe) same as Piped
• JSON (Invidious)

Why would this be useful to add?

Actually, Piped JSON format is not supported by most alternative Youtube Fronted. Which makes migration from one to another difficult.

Concept(s)

No response

Additional context

No response

Acknowledgements

• I have searched the existing issues and this is NOT a duplicate or related to another open issue.
• I have written a short but informative title.
• I filled out all of the requested information in this form.

[Bnyro]

"Actually", Piped's JSON format is the same as NewPipe's...

[noraj]

"Actually", Piped's JSON format is the same as NewPipe's...

I find out that while experimenting, the only difference is that app_version and app_version_int are void for Piped.