[DPE-3929] - feat: secrets integration

[marcoppenheimer]

NOTE: Majority of changes are synced from https://github.com/canonical/kafka-operator/pull/189. See that PR for features, refactoring and bug-fixes.

K8s Specific Changes

fix: spurious error on upgrades when container wasn't ready

• Fixed by adding a can_connect at the start of the upgrade-specific pebble-ready

  chore: drop exec failure logs down to DEBUG

• Was really noisy with the ca already exists keytool logging when re-writing the CA to the truststore

Test Changes

test: remove mTLS tests

• These weren't working, and mTLS is generally an untested feature on K8s; it's only present as a result of VM syncing
• Should be addressed in a dedicated ticket

  test: add scaling and client-broken checks to tls tests

  test: pytest.mark.unstable network_cut_without_ip_change and tls_pod_reschedule

• I suspect it's CI flakey, not a failure of the code (they pass locally)
• Let's let the user decide if they want to log out or not

  test: re-enable some commented unit tests

  test: sync unit tests with VM

• Useful feature of adding a pytestmark = pytest.mark.skipif(SUBSTRATE="k8s") to skip an entire module's tests

  test: enable upgrade int tests after stable release

• These were disabled as we didn't have a stable release, we do now, so now we have tests

  test: add consume action to app-charm for use in upgrade int tests

• Persists credentials to peer-data so that they can be used in the consumer