[MNG-7389] Incremental .m2 cache cleanup for CI

**[Thomas Skjølberg](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=skjolber)** opened **[MNG-7389](https://issues.apache.org/jira/browse/MNG-7389?redirect=false)** and commented

One or more popular continous integration are unable to properly manage the .m2 repository cache, resulting in wasted resources in the form of increased CI runtime and bandwidth consumption.

**CircleCI cache behaviour:**
- immutable cache entries
- default behaviour is to wipe the cache each time a pom file is modified (i.e. using pom hash as a cache key)
- cache entries TTL > weeks

So CircleCI always has a cache containing only the necessary artifacts, but has to download all dependencies every time the pom file changes.

**Github Actions cache behaviour**
- (effectively) mutable cache entries
- incremental cache (if it gets too big, it is wiped).
- cache entries TTL 1 week

So Github actions work well if the cache entries expire from time to time, otherwise the cache keeps growing.

**Summary**
Perhaps this does not look so bad at first glance, but for a project under active development, with a lot of artifacts, the pom file changes often. For example we have apps with 100 dependencies and automatic dependency bumping via Renovate, in addition to an hierarchy of libraries.

Key takeaways; time is wasted
- saving caches in CI
- loading cache in CI
- loading artifacts from external artifact store

This happens quite a lot. From the artifact store perspective, this probably multiplies the load by a factor of 10.

Possible solution: A way to define a "transaction" for artifact use, i.e.

1. run command to mark start of transaction
2. run one or more maven commands
3. run command to mark end of transaction, deleting artifacts not in use.

For reference, Gradle has the same problem.

Proof of concept:
* CircleCI : https://github.com/entur/maven-orb
* Github actions: https://github.com/skjolber/tidy-cache-github-action

The implementation uses instrumentation to record artifact access, then delete the artifacts not recorded. 

**Alternatives:**
I did try the last-accessed file timestamp first, turns out most CI filesystems are mounted without that option. However it should also be possible to update the modified timestamp and/or add read access to some existing metadata file. 

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No further details from [MNG-7389](https://issues.apache.org/jira/browse/MNG-7389?redirect=false)

Start with the MNG-7389 description and review the referenced maven-orb and tidy-cache-github-action proof-of-concepts. Define the scope and implementation entry points for marking artifact-use transactions, then verify that artifacts not recorded during a transaction are removed without disrupting Maven commands.

Written by the indexing model from the issue text.