Use domain in context to override RTDB domain

[fredzqm]

See go/rtdb-multi-region-function-sdk

Multi-region realtime database will be served under different domains. In order to make sure the DataSnapshot returned by the function handler can connect to the appropriate RTDB, the backend will provide domain field in the pubsub payload.

[fredzqm]

I was trying with the multi-region path. However, it didn't work due to a bug in the admin SDK.
I will fix that separately. For now, this will unblock testing in staging at least.

[fredzqm]

@laurenzlong
After talking with Ana, it looks like the data and resource are processed in some special ways that I didn't understand much.
However, any other fields in the pubsub payload will be visible as part of the context (similar to auth). The easiest and cleanest way, IMO, is to supply a domain field.

The JS SDK right now has a bug parsing the multi-region domain, but it has no problem understanding the staging domain.
I will work on a fix in the JS SDK and update the dependencies here in a follow up CL.

/cc @sarajmunjal

[fredzqm]

Move to private repos.