Adding coordinates benchmarks: search_around_sky and separation

As far as I see these are not yet covered by the benchmarks but their performance was raised as an issue in the core repo: https://github.com/astropy/astropy/issues/7358

(I'm not sure though how to address any performance benchmarking in the large dataset regime, maybe the could be "slow" benchmarks that are only run for the tag commits, etc?)

Start by reading the linked astropy issue 7358 and inspect the existing benchmark structure for coordinate operations. Add coverage for search_around_sky and separation, including a documented decision about how large-dataset or slow benchmarks should run; done means both operations are measured reproducibly.

Written by the indexing model from the issue text.