ENH: Add HEALPix and rHEALPix projections

[greglucas]

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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]

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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