Make rasterio (and GDAL) an optional dependency (move it to extras_require)

Looking at `requirements.txt`

https://github.com/geopython/pygeoapi/blob/b96765c9d49945bf3c265fe37dcae5dd343e2be7/requirements.txt#L14

We have rasterio as dependency on every `pip install pygeoapi` pulls in rasterio (that is a big GDAL binary wheel)

But rasterio is used by only three providers:

- [pygeoapi/provider/rasterio_.py](https://github.com/geopython/pygeoapi/blob/b96765c9d49945bf3c265fe37dcae5dd343e2be7/pygeoapi/provider/rasterio_.py#L34)
- [pygeoapi/provider/filesystem.py](https://github.com/geopython/pygeoapi/blob/b96765c9d49945bf3c265fe37dcae5dd343e2be7/pygeoapi/provider/filesystem.py#L262)
- [pygeoapi/provider/azure_.py](https://github.com/geopython/pygeoapi/blob/b96765c9d49945bf3c265fe37dcae5dd343e2be7/pygeoapi/provider/azure_.py#L258)

So this bloats the base install and minimal container images.

The proposed slim/distroless Docker vaiants in #1753 only avoid it via a `pip install --no-deps` workaround and explicit dependency list
```python
#Add it as an extra in setup.py:
extras_require={
'rasterio': ['rasterio'],
},
```
And remove it from requirements.txt

- Keep rasterio available to CI/tests by listing it in requirements-dev.txt (or requirements-provider.txt) so the Rasterio provider is still exercised.
- Ensure rasterio_.py degrades gracefully when rasterio is absent (the plugin loader should already surface a clear "provider unavailable" error; the two lazy importers already do the right thing).

So:
- pip install pygeoapi -> GDAL-free
- pip install pygeoapi[rasterio] - >raster/coverage support

Start with requirements.txt and setup.py to trace how rasterio enters the base install, then review requirements-dev.txt or requirements-provider.txt for CI coverage. Read pygeoapi/provider/rasterio_.py, filesystem.py, and azure_.py, including their documented lazy-import behavior. Done means the base install is GDAL-free, the rasterio extra restores raster support, and CI still exercises the provider with rasterio installed.

Written by the indexing model from the issue text.