feat(firestore): Add support for Query Partitions

[quartzmo]

Reference implementations:

• feat!: add support for the Query Partition API java-firestore#202
• feat: add Partition and BatchWrite protos nodejs-firestore#1110

Sorting logic in ResourcePath based on path.ts.

[schmidt-sebastian]

We just found out that the Partition RPC does not necessarily return the partitions in order, which can lead to overlapping results if we do not sort them ourselves. Can you add code to sort the results before building the QueryPartition? We also need to fix this in Node and Java.

[quartzmo]

Can you add code to sort the results before building the QueryPartition?

Added in new class ResourcePath, based on parts of path.ts.

[schmidt-sebastian]

As far as my Ruby expertise goes, this looks good. I did not verify the tests but the implementation handles the cases I handled in Node and Java.

[quartzmo]

It should be possible for a user to serialize the result of the QueryPartition API so that the partitions can be processed on other machines.

A reference to the original Google::Cloud::Firestore::Query instance is provided to each QueryPartition. This object holds a reference to the Google::Cloud::Firestore::Client instance, which is used in Query#get to run the query.

[quartzmo]

As discussed offline with @dazuma, serialization will be provided via new methods:

data = my_query_partition.to_json
my_query_partition = QueryPartition.from_json data, client

[dazuma]

What's here LGTM. Ping me if/when you add serialization.