Allow to release prerelease on Github

[kalosisz]

Is your feature request related to a problem? Please describe.
I would like release-please to be able to release prerelease on Github next to draft and full releases.

Describe the solution you'd like
release-please should be able to release prerelease for those versions that have the prerelease part (e.g. beta) present if a prerelease option is specified.

Describe alternatives you've considered
release-please could release all releaseses as prerelease if a prerelease option is specified.

Additional context
This feature would allow more flexibility on Github side since one could create Github Actions triggered specifilly by prerelease releases.