[cloud_firestore] Support sum() and avg() aggregation queries

[larssn]

What feature would you like to see?

Seeing as count() is already supported, it would be beneficial if sum() and avg() were also supported.
Explanations here: https://firebase.google.com/docs/firestore/query-data/aggregation-queries

[Lyokone]

PR already opened here: #11757
We are waiting for a full API Review, but if you need this feature ASAP you can checkout this branch