Create and use Firestore generic db object type

[Kayron013]

Create a generic type (E.g. DB<T>) representing an object T from Firestore, whose date fields are Firestore timestamps.
Should deprecate these:

• https://github.com/cogef/zoomer-api/blob/master/src/utils/db/types.ts#L25
• https://github.com/cogef/zoomer-api/blob/master/src/utils/db/requests.ts#L136