Support for adding DeadLetterTopic setting for programatic subscriptions

**Is your feature request related to a problem? Please describe.**

I am subscribing to the topic using a programmatic subscription as below:
```golang
sub := &common.Subscription{
PubsubName: env.GetString("PUBSUB_NAME"),
Topic: env.GetString("TOPIC_NAME"),
Metadata: map[string]string{"consumerID": "{namespace}"},
}
```
I want to specify `DeadLetterTopic` for this subscription.

**Describe the solution you'd like**

Is it possible to add `DeadLetterTopic` to `common.Subscription` structure?

**Describe alternatives you've considered**

I thought of using a declarative subscription as an alternative but I am using `grpc` hence not sure how routing will work.

**Additional context**

Start at the common.Subscription structure and trace how programmatic subscriptions are sent through the gRPC path. Confirm where DeadLetterTopic must be accepted and propagated, then verify that a subscription configured with it retains the setting; the issue does not name a test or file to run.

Written by the indexing model from the issue text.