[DPE-3136][DPE-3137] Add dpe-workflows & poetry

[phvalguima]

This PR moves the opensearch-operator to use data-platform-workflows repo, it adds more behaviors to backups, as described below; and extends its integration testing.

Moving to data-platform-workflows brings opensearch to the same level in its testing infrastructure as other operators. It also enables testing to easily consume secrets from Github, as well as other features such as redact-secrets or support for self-hosted runners. This move covers DPE-3136 and DPE-3137. Now, opensearch-operator supports Poetry and can use poetry.lock to pin dependencies.

Important movements, besides changing to dpe-workflows:

• Moves methods that are repeated several times: reset_restart_delay, c_writes, c_writes_runner and c_balanced_writes_runner to a single place: tests/integration/ha/conftest.py
• Removes tests/integration/conftest.py as the build process is now executed by the DPE workflows
• Removes the root tests/conftest.py, as it was used for secrets management between 2.9 and 3.x, besides the new workflow uses relative paths starting in tests/unit and tests/integration folders
• Moves the tests declared in tests/integration/relations/opensearch_provider/ to tests/integration/relations for the sake of simplifying relative paths
• Adds a local build of application_charm in tests/integration/relations folder

The backup test has been removed and will be readded in DPE-2999 PR.

[phvalguima]

The check is failing: app: opensearch blocked -- message: 1 or more 'replica' shards are not assigned, please scale your application up. and timing out
It is not related to this change.
Full check here: https://github.com/canonical/opensearch-operator/actions/runs/7940046438/job/21680946956?pr=178

[carlcsaposs-canonical]

thank you for migrating to data-platform-workflows! also appreciate all the bug reports & feature requests on dpw

[carlcsaposs-canonical]

poetry.lock should be version controlled

[Mehdi-Bendriss]

Thanks Pedro. I left some comments.

The base branch was changed.

[Mehdi-Bendriss]

Thanks Pedro. I left 1 comments that can be addressed in another PR