refactor: relocate json schemas out of the python package

[baha-bouali]

The JSON schemas are data, not Python code, and some of them are not
specific to config modules. Move them from cloudinit/config/schemas/
to a top-level schemas/ directory, and install them as architecture
independent data under /cloud-init/schemas.

Because the schemas no longer sit beside the code that reads them,
get_schema_dir() can no longer derive their location from file
alone. Meson records the configured install path in a generated
cloudinit/meson_paths.py, following the existing meson_versions.py
pattern. When that file is absent, as in a source checkout, the
source tree layout is used instead.

The cloudinit/config/schemas/init.py was never required and is
dropped.

Fixes GH-4688

Additional Context

Three notes for reviewers:

1. Directory name. I used schemas/ to match the install location
   agreed in refactor : Relocate json schemas #4991 (/usr/share/cloud-init/schemas).

2. The issue predates meson. It lists setup.py, which no longer
   exists after Pep632 support meson build #6326. The install is done in meson.build instead.

3. The $id URL changes. versions.schema.cloud-config.json now
   points at .../main/schemas/. The old
   .../main/cloudinit/config/schemas/ URLs will 404 once this merges,
   so a follow-up PR to SchemaStore is needed. I'd open a follow-up once
   this lands.

Test Steps

Unit tests, including three new cases covering source checkout,
installed system, and a recorded path that no longer exists:

tox -e py3 -- tests/unittests/config/test_schema.py

The install layout cannot be covered by unit tests, so verify it
manually:

meson setup builddir
DESTDIR=/tmp/stage meson install -C builddir

# schemas installed as data:
find /tmp/stage -path '*share/cloud-init/schemas*' -name '*.json'

# and no longer inside the python package:
find /tmp/stage -path '*dist-packages/cloudinit*' -name '*.json'

Then confirm a real install resolves them, with the source tree off
sys.path:

cd /tmp && PYTHONPATH=<prefix>/lib/python3/dist-packages python3 -c \
  "from cloudinit.config.schema import get_schema_dir, get_schema; \
   print(get_schema_dir()); assert get_schema()"

cloud-init schema -c user-data.yaml also still validates normally
from a checkout.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[baha-bouali]

@holmanb just a gentle ping, can you take a look at this whenever you have a chance.

[github-actions]

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