Ease the process of code versioning

[untereiner]

What is the requested feature?
Automate the code versioning process to have effective versioning of the code and maybe releases.

Is your request related to a specific problem?
I would like an easy, human readable, comparable way to discriminate versions of the public API (i.e. represented as xsd files #2971). Not sure a commit hash is the right way.

Describe the solution you'd like
I was thinking to something close to semver.

But, doing it manually is painful and time consuming.
So what about using a github-action like this one: https://github.com/ietf-tools/semver-action.
It will flow seamlessly for those using commit prefixes.
For the others, we could simply avoid the patch part and adopt a simple algorithm like this one:

If the PR needs a rebaseline (xsd change)  then bump major part
else bump minor part.

This action will make the next version available as a variable in the pipeline.
It should then be easy to patch the CMakeLists, a header file, xsd schemas, or whatever...

What to you think ?

Describe alternatives you've considered
n.a.

Additional context
n.a.

Work steps:

1. ci: Check that PR title follows conventional commit prefix #3043
2. ci: split ci into atomic workflows #3266
3. Add semver-action to compute next version based on semver
   1. Add version to code (CMakeLists, GEOS_VERSION in c++, version tag in schema)
   2. stop pushing the schema in PRs / generate it at merge step via the CI

[MelReyCG]

So, if I iterate on my branch, the version number will not change.
To improve this version number, I think we should add a build part (As semver seems to allow it).

[MelReyCG]

We should also keep this issue in mind.

[TotoGaz]

@castelletto1 What do we want to do regarding this?

[untereiner]

if #3043 is merged, it will be easy to add versions by following the semantic conventions of the merged commit messsages.
The semver-action will roll-out the commit messages between the tip and the previous release and analyse the prefixes to compute a new version number