Accept proxy environment variables from juju model config

[weiiwang01]

Configure proxy-related environmental variables automatically according to the Juju model settings.

[arturo-seijas]

LGTM

[github-actions]

Test coverage for 8edc314

Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
src/charm.py               96     17     12      4    81%   48-50, 85->exit, 99, 117-119, 135-137, 162-165, 211-212, 214-215
src/charm_state.py         93      1     10      1    98%   28
src/charm_types.py          8      0      0      0   100%
src/constants.py            6      0      0      0   100%
src/databases.py           51     15     17      2    69%   65-82, 124, 139-140
src/exceptions.py           8      1      0      0    88%   40
src/flask_app.py           21      0     12      1    97%   48->50
src/observability.py       12      0      0      0   100%
src/secret_storage.py      35      1      6      2    93%   45->exit, 74
src/webserver.py           70      4     12      1    94%   127-133, 151
-------------------------------------------------------------------
TOTAL                     400     39     69     11    88%

Static code analysis report

Run started:2023-07-28 03:43:14.729310

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2034
  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):

[yanksyoon]

LGTM!