Support for OPTIMADE v1.2.x

This issue is for tracking our support with the upcoming release candidates of OPTIMADE v1.2. There are several issues/PRs tagged under the OPTIMADE v1.2 label
and milestone.

We should collect all the PRs in a separate branch for v1.2 features before merging them into master. This will allow us to create the v1.2 OpenAPI schemas for the specification release (without necessarily having all PRs finished, fully-tested and released).

As all of the functionality in 1.2 is both optional and backwards compatible with 1.1, I don't think we necessarily need to go down the namespace package route and maintain support for older 1.1 models/servers, though it may be useful to begin thinking about it for future versions (especially the client and the validator, which could be pointed at older versions of the models for deserializing the response from other implementations). This would require an overhaul of our CI that I don't think we really have the resources to do at the moment; instead it may be easier to patch the validator/client to work differently depending on the API version returned by the implementation.

Start with the OPTIMADE v1.2 label and milestone 10, then review the issues and pull requests collected there. The scope is to coordinate v1.2 work on a separate branch and produce the v1.2 OpenAPI schemas; no specific files or tests are named in this issue.