fix: add image relation joined framework register

[yanksyoon]

Applicable spec:

Overview

• Adds image relation joined handler to framework observer.

Rationale

Juju Events Changes

Module Changes

Library Changes

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 f14df25

Name                                       Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------------------------------------
src/charm.py                                 578    139    149     26    73%   227-229, 295-314, 332-334, 335->339, 365-369, 442, 449-451, 478-483, 500-505, 526, 538-544, 558-559, 572, 577, 607-608, 610->619, 614->619, 624-630, 664, 668-673, 724-729, 738->741, 764-781, 785-786, 814-841, 854-857, 878-888, 902-904, 951-952, 954-955, 957-958, 1037->1039, 1104-1105, 1143-1145, 1153-1159, 1240-1265, 1274
src/charm_state.py                           468     17    106      5    95%   335-347, 572-576, 698-699, 754-755, 1190->1193, 1197-1198, 1245
src/errors.py                                 43      0      0      0   100%
src/event_timer.py                            54      7      2      1    86%   105-106, 131, 148-149, 165-166
src/firewall.py                               51     18     20      0    61%   42-43, 66-69, 111-185
src/github_client.py                          96     16     40      5    79%   65-72, 119->exit, 124-125, 195, 218, 231-238, 260->300, 294
src/github_type.py                            50      0      0      0   100%
src/logrotate.py                              43      0      2      0   100%
src/lxd_type.py                               35      0      2      0   100%
src/metrics/__init__.py                        0      0      0      0   100%
src/metrics/events.py                         55      2      8      1    95%   56->59, 166-167
src/metrics/github.py                         16      0      0      0   100%
src/metrics/runner.py                        144     10     32      3    91%   164, 190-203, 239, 276, 458-459
src/metrics/runner_logs.py                    24      5      4      1    79%   30-33, 47->46, 51-52
src/metrics/storage.py                        70      8     12      0    90%   89-90, 117-118, 182-183, 189-190
src/metrics/type.py                            5      0      0      0   100%
src/openstack_cloud/__init__.py               26      0      2      0   100%
src/openstack_cloud/openstack_manager.py     568    124    174     22    78%   96, 261, 436-439, 490, 493-504, 538-539, 541-542, 551, 717-744, 884-885, 907->exit, 917, 944-945, 948-949, 992, 1001-1002, 1032, 1037-1044, 1057-1063, 1067-1069, 1085-1091, 1110-1111, 1113-1124, 1131-1135, 1139-1142, 1165-1166, 1171-1177, 1195, 1210-1235, 1260-1274, 1290-1297, 1331, 1445-1450, 1471-1473, 1501-1506, 1528-1529, 1585-1586, 1596-1597
src/reactive/__init__.py                       0      0      0      0   100%
src/reactive/consumer.py                      40      2      6      0    96%   98-101
src/reactive/runner_manager.py                53      0     14      1    99%   102->exit
src/runner_manager_type.py                    51      0      8      0   100%
src/runner_type.py                            37      0     10      0   100%
src/shared_fs.py                              93     17     10      1    83%   51-52, 119-120, 145-146, 154-155, 161-162, 180, 183-184, 196-197, 240-241
src/utilities.py                              68      7     20      7    82%   86->88, 90->96, 103, 133, 147, 186-189, 244
--------------------------------------------------------------------------------------
TOTAL                                       2668    372    621     73    84%

Static code analysis report

Run started:2024-09-04 10:54:44.243128

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 9450
  Total lines skipped (#nosec): 4
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 7

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):