Create support for `Beta Program` or `Alpha Program`

[zedaraf]

Is your feature request related to a problem? Please describe.
Allow users to join FlutterMatic beta program if they are interested in early testing. This means that the default channel is stable, but they can switch between beta and alpha too.

Describe the solution you'd like
An option in settings to join the beta or alpha program.

Describe alternatives you've considered
Alternative is to download manually, build manually, and then start using. Totally inconvenient.

[yahu1031]

Need more ideas about it. Beta and Alpha stages cannot be given directly even this is an open source project. Need to rethink twice before working on this issue.