Migrate from node-fetch to axios in backend FirebaseRESTClient

[sherryhli]

Context:

• We're incompatible with node-fetch v3 (see comment in Upgrade node-fetch to v3.1.1 sistering#104)
• Though we could try to do a minor version upgrade to get the security patch, this does not seem to be a longterm sustainable solution as we may require a v3 feature one day
• Solution: migrate to Axios, with the benefit that it matches the library we use in the frontend

Acceptance Criteria:

• All node-fetch API calls are replaced with Axios equivalents in backend/typescript/utilities/firebaseRestClient.ts
• There are no breaking changes (you'll want to test the sign-in, sign-in with Google, and refresh functionalities)

Dev Notes:
uwblueprint/sistering#110