fix(postgresql): Add pg_dump

[merkata]

Overview

Adds pg_dump so Discourse backups don't fail.

Rationale

Discourse backups fail now due to missing pg_dump,

Juju Events Changes

n/a

Module Changes

n/a

Library Changes

n/a

Checklist

• [x ] The charm style guide was applied
• [x ] The contributing guide was applied
• [x ] The changes are compliant with ISD054 - Manging Charm Complexity
• [x ] The documentation is generated using src-docs
• [x ] The documentation for charmhub is updated.
• [x ] The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for 7767f77

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        326     31     82     11    90%   138, 172, 180-181, 337->342, 535-537, 542-543, 554-556, 561-562, 574-576, 581-582, 594-596, 621-623, 663->exit, 673->exit, 703-709, 735->exit, 749-750, 760->exit, 774
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               356     32     90     12    90%

Static code analysis report

Run started:2024-01-09 09:45:24.239140

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1946
  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):