Add certificates integration

[Thanhphan1147]

Applicable spec: ISD163 (internal)

Overview

Add certificates relation and certificate management logic

Checklist

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

[cbartz]

Looks good, have a few questions and nitpicks. Any plans to add tests?

[github-actions]

Test coverage for 618a484

Name                      Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------
src/charm.py                 83     53     24      0    39%   43-64, 71-72, 77-78, 83, 92-94, 103-106, 117-124, 131-132, 140-153, 157-159, 163-178
src/haproxy.py               47     14      4      1    67%   63, 71-72, 88-91, 101-103, 115-119
src/state/config.py          40     22     20      0    47%   50-70, 85-93, 105-106
src/state/tls.py             38     18     12      0    52%   53-68, 84-96
src/state/validation.py      31     17     10      0    39%   59-75
src/tls_relation.py         117     82     20      0    26%   63-76, 89-92, 100-101, 109-116, 125-143, 157-164, 177-180, 190-200, 217-239, 250, 254-255, 263-271, 279-281, 292-296, 309-313
---------------------------------------------------------------------
TOTAL                       356    206     90      1    41%

Static code analysis report

Run started:2024-09-27 11:46:20.170865

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 799
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 3

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):

[cbartz]

LGTM, except the doc string I mentioned in a comment

[Thanhphan1147]

Thanks for the reviews! I'll merge the PR now and I'll update the docstring in a follow-up PR.