Implement query_namespaces over grpc

[jhamon]

Problem

I want to maintain parity across REST / GRPC implementations. This PR adds a query_namespaces implementation for the GRPC index client.

Solution

Use a ThreadPoolExecutor to execute queries in parallel, then aggregate the results QueryResultsAggregator.

Usage

import random
from pinecone.grpc import PineconeGRPC

pc = PineconeGRPC(api_key="key")
index = pc.Index(host="jen1024-dojoi3u.svc.apw5-4e34-81fa.pinecone.io", pool_threads=25)

query_vec = [random.random() for i in range(1024)]
combined_results = index.query_namespaces(
    vector=query_vec,
    namespaces=["ns1", "ns2", "ns3", "ns4"],
    include_values=False,
    include_metadata=True,
    filter={"genre": {"$eq": "drama"}},
    top_k=50
)

for vec in combined_results.matches:
    print(vec.get('id'), vec.get('score'))
print(combined_results.usage)

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [ ] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

Describe specific steps for validating this change.