Added 'rest' and 'b2' repo support. Also added some minimal validation of mandatory repo fields

[quantum-byte]

• Adds 'rest-server' protocol support
• Adds 'b2' (Backblaze) protocol support
• Adds some minimal better error handling for repo creation/editing
• Downgrades com.android.tools.build:gradle to 7.0.4 since IntelliJ does not support it yet.
  • I can revert it but it makes my development slightly harder 😄
• Does some things with the gradle wrapper but the version stays the same (7.2)
  • Not sure what happened there but i needed to do a ./gradlew wrapper --gradle-version 7.2 to get it to build for me.

[quantum-byte]

Hi @lhns, i found this repo at the beginning of the year while looking for android backup solution with restic.
I looked nice but it did not support the Rest Server protocol yet which i wanted to use.
That is why i had a look and tried to work on it in my spare time.

I do have experience with Kotlin but this is my first Android project so please have an extra detailed look if I did anything which should not be done on Android.

Let me know what needs to be improved/changed before you can merge it. Then i ll have a look.

[lhns]

Hi! Thank you very much for your work on this. I wanted to tackle the rest-server support for a while but didn't have any time yet so I am very happy to see it working with this PR.