Shift more validation into the OpenAPI schema

Some places where we could do this (please add):

• regexps for chemical formulae (#708)
• defining an elements type to constrain strings/arrays involving elements (e.g. the elements field, species_at_sites etc.)
• provider validation should have patterns in the OpenAPI schema (#723)

Slight misdirection but there are quite a few other places that we could consider embedding validation into the OpenAPI, e.g. elements which is currently just an array of strings, could instead be an array of Elements, i.e. strings that match the regexp/enum.

Originally posted by @ml-evs in https://github.com/Materials-Consortia/optimade-python-tools/issues/708#issuecomment-780497285

Start by locating the OpenAPI schema and reviewing how the existing chemical-formula and provider patterns were added. Investigate the unchecked elements type and its uses in fields such as elements and species_at_sites; done means the relevant schema constraints are defined and covered by the project’s validation checks.