DAT-18153: Make multiplatform checks extension workflow work with PRs

[abrackx]

Keeps most of the same behavior but hardcodes less things. inputs.version was required, but there were a lot of places that had if: !inputs.version, so i removed these.

Here's an outline of what this does:

1. Check out liquibase oss and pro on appropriate branches (now workflow inputs).
2. Build liquibase oss and core to get the necessary dependencies to build liquibase-checks. We are building this without setting any versions, so this will be installed under version 0-SNAPSHOT in this workflow. The true version will correspond to the input branches.
3. Build liquibase-checks for each platform
4. Upload each of the platforms liquibase-checks jars to the workflow. << ✨ new
5. After this workflow has completed for windows, linux and macos we kick off a new workflow to combine these. This workflow effectively unzips each jar on top of each other.
6. Once we have a new multiplatform jar we set the version of this jar to the workflow's input version inputs.version. This will be the branch name in the liquibase-checks repo, but can be set via workflow dispatch. (<< ✨ new)
7. Upload this versioned jar to GPM and the workflow (<< ✨ new).

With corresponding updates to the liquibase-checks repo we now have something that should generate workflow and GPM multiplatform snapshot artifacts for both prs and merges to main.

This branch was supposed to be DAT-18153.