Support InitialSubscritionName for DLQ

[adrianotr]

Hello!

Currently the pulsar-client-dotnet api does not expose a way to create a initial subscription for dead-letter topics.
However, topics without any subscriptions do not retain messages.
Pulsar supports this already, and it seems we already have the parameter mapped in the Protobuf.

Can we add the InitialSubscriptionName attribute to the DeadLetterPolicy and send that to Pulsar during the Consumer subscription using the existing Protobuf attribute?

[adrianotr]

I've created this PR just to give you an idea of the changes needed.

[Lanayx]

Hi, @adrianotr ! Thank you for the contribution, you can continue working on PR and I'll merge it once ready. Please, don't forget to add integration test for this new feature as well.

[adrianotr]

Hi! I've pushed a new PR with an integration test: #273

[Lanayx]

Published https://www.nuget.org/packages/Pulsar.Client/3.6.0, can you please verify?

[adrianotr]

Hi @Lanayx! I've tested the new feature and it works as expected. Thanks for your support!