feat: add ChannelFinder server interfaces for SpanFE bypass

This commit adds the server abstraction interfaces for the SpanFE bypass
feature:

- ChannelFinderServer: Interface representing a Spanner server endpoint
  with address, health check, and channel access
- ChannelFinderServerFactory: Factory interface for creating and caching
  server connections
- GrpcChannelFinderServerFactory: gRPC implementation that creates and
  manages gRPC channels for different server endpoints

These interfaces enable the client to maintain connections to multiple
Spanner servers and route requests directly to the appropriate server
based on key location information.

This is part of the SpanFE bypass feature that enables location-aware
routing for improved latency.

Author: rahul2393

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/ChannelFinderServer.java

@@ -0,0 +1,11 @@
+package com.google.cloud.spanner.spi.v1;
+
+import io.grpc.ManagedChannel;
+
+public interface ChannelFinderServer {
+  String getAddress();
+
+  boolean isHealthy();
+
+  ManagedChannel getChannel(); // Added to get the underlying channel for RPC calls
+}

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/ChannelFinderServerFactory.java

@@ -0,0 +1,7 @@
+package com.google.cloud.spanner.spi.v1;
+
+public interface ChannelFinderServerFactory {
+  ChannelFinderServer defaultServer();
+
+  ChannelFinderServer create(String address);
+}

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GrpcChannelFinderServerFactory.java

@@ -0,0 +1,82 @@
+package com.google.cloud.spanner.spi.v1;
+
+import com.google.api.gax.grpc.GrpcTransportChannel;
+import com.google.api.gax.grpc.InstantiatingGrpcChannelProvider;
+import io.grpc.ManagedChannel;
+import java.io.IOException;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+class GrpcChannelFinderServerFactory implements ChannelFinderServerFactory {
+  private final InstantiatingGrpcChannelProvider.Builder channelBuilder;
+  private final Map<String, GrpcChannelFinderServer> servers = new ConcurrentHashMap<>();
+  private final GrpcChannelFinderServer defaultServer;
+
+  public GrpcChannelFinderServerFactory(InstantiatingGrpcChannelProvider.Builder channelBuilder)
+      throws IOException {
+    this.channelBuilder = channelBuilder;
+    // The "default" server will use the original endpoint from the builder.
+    this.defaultServer =
+        new GrpcChannelFinderServer(this.channelBuilder.getEndpoint(), channelBuilder.build());
+    this.servers.put(this.defaultServer.getAddress(), this.defaultServer);
+  }
+
+  @Override
+  public ChannelFinderServer defaultServer() {
+    return defaultServer;
+  }
+
+  @Override
+  public ChannelFinderServer create(String address) {
+    return servers.computeIfAbsent(
+        address,
+        addr -> {
+          try {
+            // Modify the builder to use the new address
+            synchronized (channelBuilder) {
+              InstantiatingGrpcChannelProvider.Builder newBuilder =
+                  channelBuilder.setEndpoint(addr);
+              return new GrpcChannelFinderServer(addr, newBuilder.build());
+            }
+          } catch (IOException e) {
+            throw new RuntimeException("Failed to create channel for address: " + addr, e);
+          }
+        });
+  }
+
+  static class GrpcChannelFinderServer implements ChannelFinderServer {
+    private final String address;
+    private final ManagedChannel channel;
+
+    public GrpcChannelFinderServer(String address, InstantiatingGrpcChannelProvider provider)
+        throws IOException {
+      this.address = address;
+      // It's assumed that getTransportChannel() returns a ManagedChannel or can be cast to one.
+      // For this example, GrpcTransportChannel is used as in KeyAwareChannel.
+      GrpcTransportChannel transportChannel = (GrpcTransportChannel) provider.getTransportChannel();
+      this.channel = (ManagedChannel) transportChannel.getChannel();
+    }
+
+    // Constructor for the default server that already has a channel
+    public GrpcChannelFinderServer(String address, ManagedChannel channel) {
+      this.address = address;
+      this.channel = channel;
+    }
+
+    @Override
+    public String getAddress() {
+      return address;
+    }
+
+    @Override
+    public boolean isHealthy() {
+      // A simple health check. In a real scenario, this might involve a ping or other checks.
+      return !channel.isShutdown() && !channel.isTerminated();
+    }
+
+    @Override
+    public ManagedChannel getChannel() {
+      return channel;
+    }
+  }
+}