feat(post-process-forwarder) Allow different types of post process forwarders

[nikhars]

Context

• In feat(post-process-forwarder) Different errors and transaction forwarders #28954, we created different processing strategies for errors and transactions post process forwarders.
• In this PR, we enable post process forwarder to run with one of the following entities (errors, transactions, all)
• The all option would be used in scenarios where we don't need separate post process forwarders. Example devserver, single tenant.
• There is no production change with this code since the default option for entity is "all" which means the current post process forwarder would handle all messages

[evanh]

You would need to cast this to the enum type to avoid future typing errors I think. I can't remember if mypy is smart about Enums.

[nikhars]

I opened a ticket to add mypy typing to the whole post process forwarder. I will handle it at that time.

[evanh]

Thanks!