feat: Check the URL before setting Alternative Sources

[validcube]

😊 Issue

Alternative Sources needs to check if the repository is accessible or is properly configured or not.

[!NOTE] The design is made for ReVancie☆Managerie☆Weberie! (Lotus Iteration 1), might not consistent with Flutter manager. In the context of ReVanced Manager Flutter, the feature need to check if the GitHub repository is accessible or not.

https://www.figma.com/file/hvgC1Mu2srypqC4EfaXDDL/(General-Public)-ReVancie%E2%98%86Managerie%E2%98%86Weberie!?type=design&node-id=0%3A1&mode=design&t=FV7iQ4k1iPUc5cVl-1

[!TIP] There are additional comments about Alternative Sources @ (Lesser General Public) ReVancie☆Managerie☆Weberie! (Lotus Iteration 1)

[oSumAtrIX]

Test URL could be a button somewhere on the dialog next to an Ok button. It should not prevent the user from setting that custom source. What can be done is, to check if it's working after clicking okay or whenever the app tries to contact it, so the user sees a dialog that the repo doesn't exist

[oSumAtrIX]

ReVanced Manager will soon be released as a full rewrite. Over time, many feature requests have been made for the current version of ReVanced Manager, but to accelerate the development of the new version of ReVanced Manager, we have to focus on it, which I am closing this issue according to the contribution guidelines:

Requests can be accepted or rejected at the discretion of maintainers of ReVanced Patches

This feature request has been closed for one or more of the following reasons:

• Stale for too long/ Low priority/ Maintainers un-interested/ Too complex to realize
• Feature planned/ already implemented for the new version of ReVanced Manager
• Already implemented

Regardless, anyone who comes across this feature request can PR it to the current version of ReVanced Manager until the new version is ready. Thanks for your feature request!