Add firestore admin

[leovigna]

I needed support for RxJs using FirebaseAdmin due to writing reactive code server side.
I saw this issue was brought up in the past #38

I have made a basic port for rxfire/firestore to support firestore-admin under rxfire/firestore/admin (similar to lite implementation).
The logic is essentially the same but has the following changes:

• use firebase-admin apis
• remove comparison with cache data since not applicable to firebase-admin
• remove SnapshotOptions (serverTimestamps) as not applicable to firebase-admin

I have also added a test suite that is similar to the existing one (though removed the converter tests as couldn't find documentation relating to this.

Please let me know if this PR is welcome and if there are any changes I should make to it.

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