Remove DB static config

[gtrkiller]

Remove the possibility to configure the DB with Juju config options and force the charm to use relation data.

[yanksyoon]

Minor nitpick :D LGTM!

[jdkandersson]

Looks good after comment from @yanksyoon is resolved

[github-actions]

Test coverage for 6eeb190

Name                Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------
src/charm.py          514     40    160     27    89%   199-202, 340-341, 553, 584, 630, 665-666, 717-724, 729, 831, 836-837, 909, 916, 1008, 1017, 1029, 1050, 1059, 1078, 1082, 1111, 1164, 1296, 1318, 1325->1327, 1366->exit, 1378, 1395-1397, 1415, 1424-1425
src/cos.py             15      0      0      0   100%
src/exceptions.py      17      1      2      1    89%   41
src/types_.py          15      0      0      0   100%
---------------------------------------------------------------
TOTAL                 561     41    162     28    90%

Static code analysis report

Run started:2023-08-14 13:03:31.035278

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4152
  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):