Add continuous writes test charm

[marceloneppel]

Issue

• Jira issue: DPE-532
• For at least some HA shakedown integration tests we need to ensure, along other requirements, that the PostgreSQL HA deployment correctly continues to handle incoming writes after some unexpected situations happen, like the DB process is killed, freezes, is restarted and so on.

Solution

• Add a charm that perform continuous writes to the PostgreSQL charm during for shakedown tests.

Context

In summary, this PR is a COPY-PASTE of the same code already reviewed on canonical/postgresql-operator#36, so probably doesn't need a review. But if you would like to check it, there is more context on that other PR

Testing

• This charm will be used in later PRs (like this not ready yet for review PR) to support the HA shakedown integration tests.

Release Notes

• Add continuous writes test charm (for HA shakedown tests).

[welpaolo]

LGTM