Declare the async dependency explicitly.

- The client and dispatcher require `async` directly.
- Avoid relying on `async-http` to provide it transitively.

Author: samuel-williams-shopify

async-grpc.gemspec

@@ -24,6 +24,7 @@ Gem::Specification.new do |spec|
 
 	spec.required_ruby_version = ">= 3.3"
 
+	spec.add_dependency "async", ">= 2.38.0"
 	spec.add_dependency "async-http"
 	spec.add_dependency "protocol-grpc", "~> 0.11.0"
 	spec.add_dependency "protocol-http", "~> 0.60"