[Feature Request]:[Go SDK]: BigTableIO should have scalable reads.

### What would you like to happen?

The Go BigTableIO should take advantage of BigTable providing a splitting API with the https://pkg.go.dev/cloud.google.com/go/bigtable#Table.SampleRowKeys method, in order to be authored as a Splitable Do Fn, which could then build RowSets to read from per split.

This would allow the bigtable IO to scale up it's reads.

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

- [ ] Component: Python SDK
- [ ] Component: Java SDK
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