Implements Extensions namespace

[joehan]

Implement API calls & surface for Extensions namespace, and add unit testing. To make things a little bit more palatable for review, I split this into 2 prs - please take a look at https://github.com/firebase/firebase-admin-node/pull/1829 first.

Testing: To test this, I published & installed an extension with this version of the code packaged in, and verified that it calls the correct endpoints. I wasn't able to fully test end to end yet due to issues with visibility labels on the endpoints, but I will update this once I get that working.