Change ProtoReflectionDescriptorDatabase to take grpc::ChannelInterface. (#31858)

Updates the ProtoReflectionDescriptorDatabase ctor to take a reference
to std::shared_ptr<grpc::ChannelInterface> rather than
std::shared_ptr<grpc::Channel>. This helps code that is making use of
grpc::ChannelInterface from having to perform a cast from the Base to
the Derived when creating the refelction db.

test/cpp/util/proto_reflection_descriptor_database.cc

@@ -35,7 +35,7 @@ ProtoReflectionDescriptorDatabase::ProtoReflectionDescriptorDatabase(
     : stub_(std::move(stub)) {}
 
 ProtoReflectionDescriptorDatabase::ProtoReflectionDescriptorDatabase(
-    const std::shared_ptr<grpc::Channel>& channel)
+    const std::shared_ptr<grpc::ChannelInterface>& channel)
     : stub_(ServerReflection::NewStub(channel)) {}
 
 ProtoReflectionDescriptorDatabase::~ProtoReflectionDescriptorDatabase() {

test/cpp/util/proto_reflection_descriptor_database.h

@@ -39,7 +39,7 @@ class ProtoReflectionDescriptorDatabase : public protobuf::DescriptorDatabase {
       std::unique_ptr<reflection::v1alpha::ServerReflection::Stub> stub);
 
   explicit ProtoReflectionDescriptorDatabase(
-      const std::shared_ptr<grpc::Channel>& channel);
+      const std::shared_ptr<grpc::ChannelInterface>& channel);
 
   ~ProtoReflectionDescriptorDatabase() override;