ci: fix pull request action and various CI issues

[skatsaounis]

This PR includes the following:

• Style fixes and usage of Python 3.8 to be compliant with Observability charm stack
• Introduce of static tests for both charm and library code to conform to Observability Github actions
• Re-publishing of the charm library under the proper charm. The existing LIBID was causing 500s on fetch-lib, publish-lib.
• Implementation of integration tests
• Usage of lxd as the provider of integration tests.

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[alexsander-souza]

+1