Support creating repos if not exist

[oliversun9]

Add optional key create_visibility to action, specifying it adds --create --create-visibility to buf generate invocation.

Note that CLI requires both --create and --create-visibility, whereas this action only requires create_visibility.

I prefer it this way because GitHub action error feedback loop is longer than the CLI. In an action, a user modifies the workflow file, commits it, pushes, waits for the action to run, checks result (a few clicks), sees an error. On the CLI, a user types on the command line and sees the result right away.

It is easier for the user to get it right on the first try if we only require one field.

I am also open to require create as well, for consistency with the CLI.

[doriable]

This looks good to me, let some comments and a couple of notes:

• should add to the README
• I agree that we can just have the create_visibility input to capture both --create and the visibility.

[cyinma]

generally lgtm with few nits, in practice, creating resources require at least organization admin role, and I wonder how many user will put an admin token into the Github action, but this still feel useful to me