gh-138633: indicate SSL backend requirements for SSL functions

[picnixz]

If we are going to support multiple flavors of libssl, we should also plan to ease our life in the future when we want to say "this is ok for this flavor" so I added a Sphinx extension (actually it's similar to the availability directive but I decided to have a separate directive as it's not really the same; since the code would be the same, I just made a base directive class for that)

• Issue: gh-138633

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