CI: Extract changelog section

[bisgardo]

Use new Action for extracting the appropriate changelog section of the library to release. The release notes are constructed from the message of an annotated tag followed by this section.

The strategy of cloning to a temporary location and building from the subfolder has been reverted because it didn't respect the lock file. This unfortunately means that we sometimes build against a local copy of wallet-connectors which may be unreleased instead of forcing the use of a released version. I haven't yet been able to find a solution to that problem.

Some of the steps are still too ugly; it may be worth extracting one or two more custom actions before expanding this to other projects.

[bisgardo]

I would probably cherry-pick/squash the commits here 🙃

I (almost) always do :)

[bisgardo]

I would probably cherry-pick/squash the commits here 🙃

I (almost) always do :)