Add staging signing config

[Chrylo]

closes #27

• The release build type is now signed with the keystore added to the org secret
• Added another unsigned build type so we still have an unsigned artifact

You can this this in your own repo when using the same keys like this:

Can be tested locally when adding e.g. the following to the local.properties: release.keystore.path=.android/debug.keystore release.keystore.key.alias=androiddebugkey release.keystore.key.password=android release.keystore.store.password=android

Docs for composite action: https://docs.github.com/en/actions/creating-actions/creating-a-composite-action

Added GitHub action for decoding the base64 keystore file: https://github.com/timheuer/base64-to-file

[wba2hi]

@SebastianSchildt PR can be merged