Expose databaseId and projectId on Firestore class

[IchordeDionysos]

Given the introducing of multiple databases it becomes more and more often that you want to know from where a Reference or snapshot was created for.

Right now there is no way to go from a DocumentReference or a DocumentSnapshot to the database or the project.

It should simply be possible to do the following:

reference.firestore.databaseId
reference.firestore.projectId

Those getters already exist but they are marked a private/internal making accessing them in Typescript unavailable.

Right now you need to do some fishy stuff accessing internal variables, silencing Typescript errors (using as any) or other unofficial, undocumented things.

Given those things are not officially supported and APIs may change without proper breaking changes this makes depending on those workarounds, not a solution.

[IchordeDionysos]

@tom-andersen with Multi-DB support, I think this is essential to effectively work with it.
And it should be a small easy change to get approved in your internal API change (I hope at least 🥹)

I checked the other SDKs (Android, iOS) that allow to retrieve the settings again (which the NodeJS SDK does not), so it could be an option to do:

reference.firestore.settings.databaseId
reference.firestore.settings.projectId

There would be naming conflict though between the settings getter and the existing setter.

/cc @wu-hui