feat(db-sync): Vault's database secrets engine

[nrdxp]

Adds support to retrieve credentials dynamically from Vault's database secrets engine, assuming the user has enabled and configured it. There is also a new function which renews the credentials lease every two minutes to avoid a situation where the credentials eventually expire.

This change also maintains the legacy path as well.

The link is still private for many, but a real world example of how to configure this nicely in terraform lives here.

[nrdxp]

Converting to draft since I just hit hashicorp/nomad#15714

We may just have to renew the lease ourselves after all. Reopened with lease renewal accounted for in the entrypoint.

[nrdxp]

Okay this is finally ready, working and tested. When a credentials lease hits its max ttl and can no longer be extended the entrypoint will now stop the background db-sync process, grab brand new credentials, and then restart the background db-sync, allowing the job to run indefinitely.

Also I added a snippet to exit entirely if db-sync stops unexepectedly, since it has happened before and causes and outage, where now Nomad will simply reschedule and allow the work to continue.

[nrdxp]

I found out today that with more appropriate SQL creation statements, we can avoid having to manage the lease manually at all, so I can simplify this PR quite a bit before merging, putting in draft til then.