feat(ISD-847): Prevent server_name to be changed

[amandahla]

Prevent server_name to be change by comparing with the one in the configuration file if exists.

[yanksyoon]

Very nice!

[github-actions]

Test coverage for d7e3bb5

Name                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------
src/charm.py            62      2      6      1    96%   118-119
src/charm_state.py      38      2     10      2    92%   22, 63
src/charm_types.py       6      0      0      0   100%
src/constants.py        10      0      0      0   100%
src/exceptions.py       10      0      0      0   100%
src/synapse.py          58      2      8      1    95%   133-138
----------------------------------------------------------------
TOTAL                  184      6     24      4    95%

Static code analysis report

Run started:2023-06-28 18:59:25.616224

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 975
  Total lines skipped (#nosec): 0
  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):

[gtrkiller]

LGTM

[yanksyoon]

LGTM!