Fix CRS support on WMS Facade

[doublebyte1]

Overview

Related Issue / discussion

#2301

Additional information

This PR enables support to WebMercator crs in the WMS Facade provider. It also enables users to express crs in a number of different ways, other than the uri; although this is not in the Standard, it is provided as a matter of convenience.

The related docs are also updated to reflect this. I will add unit tests on a separate PR.

With this update, we can send a request for WGS84:
http://localhost:5000/collections/mapserver_world_map/map?crs=EPSG:4326&f=png&bbox=-20037508.34,-20037508.34,20037508.34,20037508.34&bbox-crs=3857

And get a different map from a request for WebMercator:
http://localhost:5000/collections/mapserver_world_map/map?crs=EPSG:3857&f=png&bbox=-20037508.34,-20037508.34,20037508.34,20037508.34&bbox-crs=3857

Some things to take into consideration for the future:

• Adding support for any crs (not easy, because the crs_transform functions and the WMS provider take the crs in different formats).
• Review the MapScript provider to ensure the behaviour is uniform.

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines