Refactor to adapt paas-app-charmer for new frameworks

Applicable spec:

Overview

This PR consists in a refactor to adapt paas-app-charmer for new frameworks. In concrete, for Go and FastAPI.

As such, the main changes are:

The _gunicorn.charm.GunicornBase class is stripped base charm, and now it will inherit from PaasCharm, which will contain the common functionality to all the charms.
PaasCharm will be now the base charm. Nearly all the frameworks will have the same functionality, and they will differ mostly in how they prepare the configuration for the App, and the framework specific configuration.
PaasCharm init is reduced a bit more and made a bit more generic.
_gunicorn.secret_storage.GunicornSecretStorage is renamed to KeySecretStorage, as we have not found any specific framework use of this class, it will be generic.
Most of the _gunicorn.app.WsgiApp will go to a base class name App. _gunicorn.app.WsgiApp will container Gunicorn specific things related to the webserver.
Many other minor refactorings and renamings to suit the new structure.
Integrations tests now run against juju 3.5 and the new charmcraft/rockcraft extensions for Go. Fixes for charmcraft files that required rust.

Rationale

After this PR, the implementation of Go and FastAPI will be straightforward.

Juju Events Changes

Module Changes

Library Changes

Checklist

[x] The charm style guide was applied
[x] The contributing guide was applied
[x] The changes are compliant with ISD054 - Managing Charm Complexity
[x] The documentation is generated using src-docs
[x] The documentation for charmhub is updated
[x] The PR is tagged with appropriate label (urgent, trivial, complex)

Test coverage for 72878b1b4541830b536c6c53c2ca03fc438ad6b0

Name                                            Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------------------------------
paas_app_charmer/__init__.py                       29     14      0      0    52%   13-14, 19-20, 26-27, 33-34, 40-41, 47-48, 54-55
paas_app_charmer/_gunicorn/__init__.py              0      0      0      0   100%
paas_app_charmer/_gunicorn/charm.py                15      0      0      0   100%
paas_app_charmer/_gunicorn/webserver.py            76      4     14      1    94%   162, 174-180
paas_app_charmer/_gunicorn/workload_config.py       8      0      0      0   100%
paas_app_charmer/_gunicorn/wsgi_app.py             18      0      0      0   100%
paas_app_charmer/app.py                           120      0     48      2    99%   94->exit, 135->141
paas_app_charmer/charm.py                         212     25     46      4    88%   32-33, 40-41, 190-191, 193-194, 215->exit, 227-231, 284-286, 339-340, 345, 350, 355, 365, 370, 375, 380, 385, 410
paas_app_charmer/charm_state.py                   107      2     20      2    97%   184, 268
paas_app_charmer/charm_utils.py                    23      0      0      0   100%
paas_app_charmer/database_migration.py             35      0      2      0   100%
paas_app_charmer/databases.py                      25      2     11      1    92%   89-90
paas_app_charmer/django/__init__.py                 2      0      0      0   100%
paas_app_charmer/django/charm.py                   34      4      6      2    85%   44, 81, 96-97
paas_app_charmer/exceptions.py                      5      0      0      0   100%
paas_app_charmer/flask/__init__.py                  2      0      0      0   100%
paas_app_charmer/flask/charm.py                    24      0      0      0   100%
paas_app_charmer/observability.py                  16      0      4      1    95%   40->44
paas_app_charmer/secret_storage.py                 50      3     16      5    88%   51, 55->54, 56->58, 86, 105
paas_app_charmer/utils.py                          11      2     12      2    65%   29, 31
-------------------------------------------------------------------------------------------
TOTAL                                             812     56    179     20    92%

Static code analysis report

Run started:2024-08-14 12:53:06.810924

Test results:
    No issues identified.

Code scanned:
    Total lines of code: 1760
    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):

canonical / paas-app-charmer