Improve datastore type hints

This issue has been migrated from [#14678](https://github.com/matrix-org/synapse/pull/14678).

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By being explicit about methods we *should* be able to get better type checking (although this isn't entirely proven in this PR). The idea is to mostly treat datastores the way we handle config classes, each as an individual unit that has to reach back to a "root" object to query other datastores.

This means that the main `DataStore` object would become a bundle of `SQLBaseStore` objects instead of a single entity which has super-classes of `SQLBaseStore`.

As a proof-of-concept I converted the `RelationsStore` (it is simple and I know it fairly well). Other stores could be done in follow-ups.

Related to #11165

Downsides of this approach is it might make #11733 worse as you now have a `DataStore` which includes a `RelationsStore`? It adds another layer to the puzzle.

Start by reviewing the existing RelationsStore proof of concept and how datastore classes are currently structured, then compare that with the config-class approach mentioned in the issue. A concrete follow-up scope, affected stores, validation strategy, and definition of done are not specified here.

Written by the indexing model from the issue text.