Create SECURITY.md

[jdkandersson]

Overview

Adds the SECURITY.md file with how to disclose a security vulnerability.

Rationale

This will enable users to report security issues to us

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The changes do not introduce any regression in code or tests related to LXD runner mode.

[github-actions]

Lint checks failed for 238036a

```

tests/integration/conftest.py:576: error: Item "Type" of "Type | None" has no attribute "units" [union-attr]
tests/integration/conftest.py:576: error: Item "None" of "Type | None" has no attribute "units" [union-attr]
Found 2 errors in 1 file (checked 57 source files)

[github-actions]

Test coverage for 238036a

Name                         Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------
src/charm.py                   655    150    140     27    74%   254-256, 322-341, 359-361, 362->366, 392-396, 470-472, 481, 488-490, 511-516, 533-538, 559, 571-577, 592-593, 612-613, 622, 627, 657-658, 660->669, 664->669, 674-681, 715, 719-724, 776, 788->791, 814-826, 830-831, 864-865, 877-894, 918-920, 939-949, 1029-1030, 1032-1033, 1035-1036, 1115->1117, 1182-1183, 1221-1223, 1231-1239, 1315-1348, 1362-1367, 1382-1425, 1433-1434, 1456
src/charm_state.py             450     17     82      3    95%   274-286, 505-509, 631-632, 687-688, 1123->1126, 1130-1131, 1178
src/errors.py                   25      0      0      0   100%
src/event_timer.py              52      6      0      0    88%   105-106, 143-144, 160-161
src/firewall.py                 51     18     10      0    67%   42-43, 66-69, 111-185
src/github_client.py            23      2      4      0    93%   71-72
src/logrotate.py                43      0      2      0   100%
src/lxd_type.py                 35      0      0      0   100%
src/runner_manager_type.py      39      0      0      0   100%
src/runner_type.py              38      0      0      0   100%
src/shared_fs.py                98     17     10      1    83%   60-61, 132-133, 162-163, 171-172, 178-179, 210, 213-214, 226-227, 270-271
src/utilities.py                32      4      6      2    79%   66-69, 111
------------------------------------------------------------------------
TOTAL                         1541    214    254     33    84%

Static code analysis report

Run started:2025-01-10 11:40:52.004971

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 5084
  Total lines skipped (#nosec): 2
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 6

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):