ENH: Add HEALPix and rHEALPix projections

[greglucas]

Rationale

This adds the HEALPix and rHEALPix projections from PROJ to Cartopy.
https://proj.org/en/9.4/operations/projections/healpix.html
https://proj.org/en/9.4/operations/projections/rhealpix.html

This replaces #1008 and allows for flexible globes/radii as well.

closes #882
closes #1008
closes #2133

[greglucas]

For class names, do we want Healpix() or HEALPix()? Similarly for the rectangular version, do we want Rectangular spelled out, or is RHEALPix as I've put it ok?

[rcomer]

Also closes #2133 I think.

[lgolston]

Always enjoy seeing new projections be added! For the name, I would vote for HEALPix() and rHEALPix().

[greglucas]

Note that I added this here because I don't have PROJ 9.6 installed locally, so this didn't work when instantiated. Maybe there is a better way to approach this?

[greglucas]

OK, I decided to upgrade my local PROJ and rebuild this file instead. I think it is valuable to keep the Spilhaus image in there. I'm not sure if there is still a better way to do this and allow building this file without all of the projections available...

[greglucas]

Always enjoy seeing new projections be added! For the name, I would vote for HEALPix() and rHEALPix().

I think I prefer the capitalization of "R" to indicate it is a class in Python by common convention, even if PROJ does lowercase it. Any other votes/preferences on naming?

I think this is ready for review again, and I added in a review comment about the doc build process when someone doesn't have the latest PROJ installed.

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