feat: support count optimization with indexed columns

[jackye1995]

This PR adds support for count(*) with a column that is indexed:

1. if the index is up to date, directly count rows using the index locally
2. if the index is not up to date, do a split plan to distributedly count individual fragments, and have a single worker count the indexed fragments using the index

This could be extended to technically other use cases, will do it in follow up PRs

Pending merge of lance-format/lance#5447

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