[FEATURE] Add GRPC support for k-NN queries

[karenyrx]

Is your feature request related to a problem? Please describe.

Currently, the k-NN plugin only supports REST API queries. With OpenSearch moving transport-grpc from a plugin to a core module (PR #18897), and real world benchmarking results (which will be published in a separate future issue) showing a sizeable performance gains for GRPC KNN over HTTP queries, there's an opportunity to provide native GRPC support for k-NN queries.

Describe the solution you'd like

Add native GRPC support for k-NN queries, enabling clients to perform vector searches using Protocol Buffers over GRPC transport instead of REST APIs.

Expected performance benefits include:

• Reduced Serialization Overhead: Binary protocol buffers vs JSON
• Lower Network Latency: HTTP/2 multiplexing vs HTTP/1.1
• Better Throughput: Connection reuse and streaming capabilities
• Type Safety: Compile-time validation vs runtime JSON parsing
• Streaming support: Potential for streaming large result sets

Describe alternatives you've considered

N/A

Additional context

Milestones

• Core PR to allow the transport-GRPC to be extensible
• k-NN PR to implement the KNN GRPC Support by extending the transport-grpc plugin
• Add user documentation for GRPC k-NN queries, under https://docs.opensearch.org/latest/api-reference/grpc-apis/search/
• Publish issue benchmarking results of GRPC KNN compared to REST API