vecindex: add SearchForInsert to vector index library

[andy-kimball]

Add a SearchForInsert method to the VectorIndex class that searches for the best partition in which to insert a query vector. This will be used by the execution engine, which creates its own custom insertion batches rather than using VectorIndex.Insert. Add a GetPartitionMetadata method to the Store class that's used by SearchForInsert to efficiently get the centroid of the insert partition, as well as to check whether the partition is over-sized.

Epic: CRDB-42943

Release note: None

[cockroach-teamcity]

This change is 

[DrewKimball]

with nits.

Reviewed 12 of 12 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mw5h)

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pkg/sql/vecindex/vecstore/store.go line 112 at r1 (raw file):

	// ErrRestartOperation if the caller should retry the operation that triggered
	// this call.
	GetPartitionMetadata(

When would we call this with forUpdate=false? Can we simplify the method and have it always take a shared lock?

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pkg/sql/vecindex/vecstore/search_set.go line 168 at r1 (raw file):

	// candidates for reranking and statistics (up to MaxExtraResults) that are
	// within the error threshold of being among the best results.
	// Note: When ForInsert mode is enabled, MaxResults must be set to 1.

nit: there's no ForInsert mode; this comment seems stale.

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pkg/sql/vecindex/vecstore/search_set.go line 175 at r1 (raw file):

	// statistics. These results are within the error threshold of being among the
	// the best results.
	// NOTE: When

nit: incomplete comment.

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pkg/sql/vecindex/vector_index.go line 370 at r1 (raw file):

// It always returns a single search result containing the key of that
// partition, as well as the centroid of the partition (in the Vector field).
func (vi *VectorIndex) SearchForInsert(

Would it be possible to call this method as part of Insert()? Also, could we do something similar for deletes?