Support Adding Additional Changelog Types

[sam3k]

TL;DR

I need to generate a changelog without PR links for each commit and would like to do so by adding my own changelog-type as described here.

Detailed design

Docs mentioned that we can use ChangelogNotes interface to add additional changelog types; but this seems to not apply to the Github action.

Additional information

No response

[chingor13]

See related: https://github.com/googleapis/release-please/issues/2080

[danielo515]

It will be nice if they provide some clear and concise instructions about how to do things. As by most google products, the existing documentation is ambiguous and it is not clear if some things are even possible.