ISD-2586 Support for ha in active/passive mode

[Thanhphan1147]

Spec: ISD-196 (internal)

This PR adds support for the ha integration to be used with the hacluster charm to provide high-availability support through pacemaker/corosync, along with unit and integration tests.

Supported workflow:

juju deploy haproxy -n 3
juju deploy hacluster
juju config haproxy vip=<vip>
juju relate haproxy hacluster

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)

[github-actions]

Test coverage for 93f3dae

Name                      Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------
src/charm.py                156     64     28      5    53%   78, 136-137, 142, 147-149, 157-170, 175, 180, 193-228, 239, 254-257, 264, 277-279, 289-294, 299, 311-325
src/haproxy.py               65     17      2      1    73%   73, 90-95, 110-117, 125-131, 140-149, 159-161, 189
src/http_interface.py        73     25      4      0    62%   74, 83, 92, 106-108, 126, 138, 150, 162, 170-175, 187, 194, 202, 217-227
src/state/config.py          41     22      6      0    40%   52-72, 87-95, 107-108
src/state/exception.py        1      0      0      0   100%
src/state/ha.py              30      1      2      1    94%   50
src/state/ingress.py         35      0      4      0   100%
src/state/tls.py             34     10      6      1    62%   51-59, 85-90
src/state/validation.py      31      6      6      2    73%   62-67, 69->72, 72->74
src/tls_relation.py          53     12     20      9    71%   59, 61, 63, 69-70, 75-76, 78-79, 80->exit, 100-102, 111->113
---------------------------------------------------------------------
TOTAL                       519    157     78     19    65%

Static code analysis report

Run started:2025-01-21 11:52:19.392598

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2329
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 2

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):