Abstract analytics service

[edeleastar]

The Analytics service:

• analytics.ts

is currently bound to firebase real time database implementation. Introduce an abstraction layer to facilitate the replacement of the firebase strategy with alternative data store implementation.

This will need to be accompanied by a companion abstraction to enable the data store contents to be read and presented in TutorsTime. Current implementation is tied to firebase:

• metrics.ts
• [https://github.com/tutors-sdk/tutors/blob/development/src/lib/services/utils/metrics.ts]