feat: manual affinity via call options or context

[nimf]

This feature allows specifying affinity key via context or call options.

The key from call options takes precedence over key in context if both specified.

Examples:

Using gRPC context:

import io.grpc.Context;

...

Context ctx = Context.current().withValue(GcpManagedChannel.AFFINITY_CTX_KEY, "your-affinity-key");
ctx.run(
    () -> {
      stub.executeStreamingSql(executeSqlRequest, resp);
    });

Using gRPC call options:

stub.withOption(GcpManagedChannel.AFFINITY_KEY, "your-affinity-key")
    .executeStreamingSql(executeSqlRequest, resp);