Use the new database interface for the postgresql relation

[nrobinaubertin]

I created a database.py module alongside the charm.
It uses the new postgresql_client interface.

[amandahla]

LGTM

[yanksyoon]

Looks great! Minor nitpicks as comments, would it be correct to assume that the stylistic issues are pretty hard to overcome at this stage of the charm?

[github-actions]

Test coverage for 92ed5be

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        242     16     68      7    93%   126, 135, 143, 420-421, 462-464, 507-509, 512->exit, 522->exit, 557-563, 589->exit, 603-604
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               272     17     76      8    93%

Static code analysis report

Run started:2023-07-28 13:50:42.417790

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1715
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[mthaddon]

LGTM - we'll want to update the docs to direct people to use this new relation and charm, but that can happen as a later step.

[amandahla]

LGTM