[api] Make strategies injectable

[Jipperism]

Make strategies injectable so they can be resolved directly in files such as [src/lib/users/UserUpsertStrategy.ts](https://github.com/hypercerts-org/hypercerts-api/pull/263/files/fab4d466c62228baffaf8321ad57c14c58544893#diff-c6212a6a5dd6ff026ea3aae8cbc42f155730969ba3da2a745dd4b27476c30cde).

See #263 (comment).