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

[jorgejesus]

Looking at requirements.txt

pygeoapi/requirements.txt

Line 14 in b96765c

rasterio

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
  • pygeoapi/provider/filesystem.py
  • pygeoapi/provider/azure_.py

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

 #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

[webb-ben]

Its worth aligning on how pygeoapi declares dependencies. Probably should not be spreading across setup.py and requirements.txt files. Related to #2269