Expose future stub on Pinecone connection (#32)

This commit exposes the future stub on Pinecone connection, allowing for
non-blocking async calls

## Problem

PineconeConnection does not currently expose a stub for making
asynchronous calls to the service. Async calls are very common in high
traffic services.

## Solution

Exposed the generated `VectorServiceGrpc.VectorServiceFutureStub` on
`PineconeConnection`. This returns
`com.google.common.util.concurrent.ListenableFuture<V>` for each call,
which allows chaining of asynchronous operations.

The PR also removes an unused public method, `setBlockingStub`; can add
this back if client API compatibility dictates.

## 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

Run `sanityAsyncFuture` test

Author: russcam

src/main/java/io/pinecone/PineconeConnection.java

@@ -34,7 +34,7 @@ public class PineconeConnection implements AutoCloseable {
      */
     private VectorServiceGrpc.VectorServiceBlockingStub blockingStub;
 
-    private VectorServiceGrpc.VectorServiceStub asyncStub;
+    private VectorServiceGrpc.VectorServiceFutureStub futureStub;
 
     public PineconeConnection(PineconeClientConfig clientConfig, PineconeConnectionConfig connectionConfig) {
         this.connectionConfig = connectionConfig;
@@ -45,13 +45,17 @@ public PineconeConnection(PineconeClientConfig clientConfig, PineconeConnectionC
                 ? connectionConfig.getCustomChannelBuilder().apply(clientConfig, connectionConfig)
                 : buildChannel(clientConfig, connectionConfig);
         channel.notifyWhenStateChanged(channel.getState(false), this::onConnectivityStateChanged);
-        Metadata metadata = assembleMetadata(clientConfig, connectionConfig);
+        Metadata metadata = assembleMetadata(clientConfig);
         blockingStub = VectorServiceGrpc
                 .newBlockingStub(channel)
-                .withInterceptors(MetadataUtils.newAttachHeadersInterceptor(metadata));
-        asyncStub = VectorServiceGrpc
-                .newStub(channel)
-                .withInterceptors(MetadataUtils.newAttachHeadersInterceptor(metadata));
+                .withInterceptors(MetadataUtils.newAttachHeadersInterceptor(metadata))
+                .withMaxInboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE)
+                .withMaxOutboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE);
+        futureStub = VectorServiceGrpc
+                .newFutureStub(channel)
+                .withInterceptors(MetadataUtils.newAttachHeadersInterceptor(metadata))
+                .withMaxInboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE)
+                .withMaxOutboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE);
         logger.debug("created new PineconeConnection for channel: {}", channel);
     }
 
@@ -79,8 +83,8 @@ public VectorServiceGrpc.VectorServiceBlockingStub getBlockingStub() {
         return blockingStub;
     }
 
-    public void setBlockingStub(VectorServiceGrpc.VectorServiceBlockingStub blockingStub) {
-        this.blockingStub = blockingStub;
+    public VectorServiceGrpc.VectorServiceFutureStub getFutureStub() {
+        return futureStub;
     }
 
     private void onConnectivityStateChanged() {
@@ -104,20 +108,13 @@ public static ManagedChannel buildChannel(PineconeClientConfig clientConfig,
         return builder.build();
     }
 
-    private static Metadata assembleMetadata(PineconeClientConfig clientConfig,
-                                             PineconeConnectionConfig connectionConfig) {
+    private static Metadata assembleMetadata(PineconeClientConfig clientConfig) {
         Metadata metadata = new Metadata();
         metadata.put(Metadata.Key.of("api-key",
                 Metadata.ASCII_STRING_MARSHALLER), clientConfig.getApiKey());
         return metadata;
     }
 
-    private VectorServiceGrpc.VectorServiceBlockingStub applyDefaultBlockingStubConfig(VectorServiceGrpc.VectorServiceBlockingStub stub) {
-        return stub
-                .withMaxInboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE)
-                .withMaxOutboundMessageSize(DEFAULT_MAX_MESSAGE_SIZE);
-    }
-
     static String getEndpoint(PineconeClientConfig clientConfig, PineconeConnectionConfig connectionConfig) {
         String endpoint = (connectionConfig.getConnectionUrl() != null) ?
                 connectionConfig.getConnectionUrl().replaceFirst("https?://", "") :