[Feature] Implement pruning for neural sparse search

[zhichao-aws]

Description

Implement prune for sparse vectors, to save disk space and accelerate search speed with small loss on search relevance. #946

• Implement pruning at sparse_encoding ingestion processor. Users can configure the pruning strategy when create the processor, and the processor will prune the sparse vectors before write to index.
• Implement pruning at neural_sparse 2-phase search. Users can configure the pruning strategy when search with neural_sparse query. The query builder will prune the query before search on index.

Related Issues

#946

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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