feat(extensions): support filtering by base

[gcomneno]

Fixes #4360.

This adds a --base option to snapcraft extensions, matching the existing snapcraft plugins --base behaviour.

With this change, users can list only the extensions supported by a specific base, for example:

snapcraft extensions --base core24

Changes:

• add --base to the extensions command
• filter listed extensions by supported base when provided
• reject unsupported and maintenance bases
• add unit coverage for base filtering and errors

Validation:

• pytest -q tests/unit/commands/test_list_extensions.py -rs
• ruff check snapcraft/commands/extensions.py tests/unit/commands/test_list_extensions.py

[gcomneno]

The remaining failures look unrelated to this PR. The Spread failure is in tests/spread/core24/try, while this PR only adds a --base option to snapcraft extensions and updates unit tests. QA/lint and the targeted unit tests are passing.