Refactor Postgresql::ReplicationSlot to support multiple databases

What does this MR do?

This refactors Postgresql::ReplicationSlot so it can be used with multiple primary databases, as part of our effort to split data (e.g. CI tables) across different databases.

The work is broken up in a few commits, each taking a distinct/isolated step towards multiple database support. For more information on those specific steps, refer to the commits and their messages.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • This is a backstage-only change, so no changelog is needed
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.