Add test_functional test

[BON4]

Issue #470

Functional Testing. Recovery testing.

This pull request is based on the same PR but for VM postgresql-operator

All this cases should be covered:

• Graceful restart. DB will receive SIGTERM and will shutdown properly . Write data, graceful restart instance.
• Forceful restart. DB will receive SIGKILL and will shutdown forceful, on start charm/patroni should recover. Write data, forceful restart instance (recovery).
• Backup-restore case with recovery. Write data, create backup, forceful restart, run recovery, write data, restore, check data consistency.

[taurus-forever]

Abandone for now. No success story here, unfortunately. I hope one way we have it merged on the next Jubilant iteration.