Parallelize charm builds in integration tests

[NohaIhab]

Context

In the CI of this repo, the integration tests are packing up to 8 charms in a single GH runner. This is problematic because it is very time consuming and causing pressure on disk space. It is often the cause of the tests failing in the CI.

To solve this, we can parallelize the building of the charms by having each charm build on a separate runner, then pass the built .charm files to the tests through GH artifacts.

A PoC has been implemented in #571, that PR can be used as a reference.

What needs to get done

1. Refactor the CI to pack each charm in a GH runner
2. Refactor the CI to pass the packed .charm files to the tests job
3. Refactor the tests job to use the input .charm files to deploy instead of packing the charms in one runner.

Definition of Done

Charm builds are parallelized and reused across tests in the CI

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-6638.

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