repo-store: Add Create method

Created by: ryanslade

In internal/db/repos:

Add a Create method:

func (s *repos) Create(ctx context.Context, repos ...*types.Repo) error

We should hopefully just be able to copy what's here: https://github.com/sourcegraph/sourcegraph/blob/e7490b028389f39e24e4fa7ec5ca87b4d4450a6b/cmd/repo-updater/repos/store.go#L427

Extracted from: https://github.com/sourcegraph/sourcegraph/issues/14092