Add proxy juju env support

[nrobinaubertin]

Overview

Add support for juju-http(s)-proxy model-config settings.
This populates the http_proxy, no_proxy environment variables for all pebble operations.

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 ab366c4

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        325     31     82     11    90%   137, 171, 179-180, 336->341, 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               355     32     90     12    90%

Static code analysis report

Run started:2023-12-04 16:02:47.959044

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):