Add type annotation checks to CI

[plamut]

We do not run static type checks on the code, but we probably should, which will allow us to safely incrementally annotate the library code, i.e. without incorrect annotation that would break users' checks.

Something similar to the pull request that we merged a few months ago in BigQuery.

(the scope of the issue can be extended to also include annotating all public methods) Will be done separately.