Add NATS request/reply pattern processor

[marshauf]

Adds a Benthos processor component, which allows sending request to a NATS subject and awaits a response.

Based on the concept of https://docs.nats.io/nats-concepts/core-nats/reqreply

Developed alongside #2352 and I am also happy to hear any feedback.

[insidegreen]

Hi @marshauf,

great stuff! I'd tested your PR ... works great!
Could you add a parameter to set the InboxPrefix on the NATS connection?

And one question: You're using a mutex to lock the NATS connection?! But as far as I can see, the NATS connection does the same internally ...
Is there another reason for your mutex?

[marshauf]

Hi @insidegreen ,
thank you. No, I will most likely not do anything more, than maintenance for my PRs until they get accepted. If you add the functionality I can add it to this PR.

Yes, the reason is in the Close method. When Benthos shuts down or decides to reconnect, the NATS connection gets dropped and deleted/set to nil. The mutex avoids calling a method on a nil NATS Connection in the Process method.
Since I am not 100% sure Benthos will not call the Close method until all calls to the process Method return and it will not call the process Method until Close and Connect return, I added the mutex to be on the safe side.

[insidegreen]

Hey @marshauf ,

customer inbox prefix for the processor
chi-deutschland#1

[Jeffail]

LGTM, thanks @marshauf (and @insidegreen)