Allow users compute statistics over retrieved batch datasets

[zhilingc]

What this PR does / why we need it:
This PR adds the functionality to allow users to optionally compute tfdv statistics over retrieved batch datasets:

# compute_statistics defaults to false. If false, statistics() will always return an empty object.
dataset = client.get_batch_features(feature_refs, entity_rows, compute_statistics=True)
# waits for retrieval job to be complete, then returns tfdv statistics
stats = dataset.statistics()

Like with the feature set statistics, these statstics can then be fed into tfdv for validation, or facets to explore the dataset.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

- Adds the parameter compute_statistics to client.get_batch_statistics()
- Adds the method statistics() which returns statistics from retrieval job

[zhilingc]

/test test-end-to-end-batch

[zhilingc]

/test test-end-to-end-batch

[zhilingc]

/test test-end-to-end-batch

[zhilingc]

/test test-end-to-end-batch

[zhilingc]

/test test-end-to-end-redis-cluster

[woop]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: woop, zhilingc

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• OWNERS [woop,zhilingc]

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[woop]

/kind feature