limits support for target.smtp

[foxcpp]

Use case

Forwarding messages to external systems with ratelimits.

Your idea for a solution

Add limits to target.smtp - the code will be very similar to target.remote.

As an alternative consider implementing target-agnostic "target.limited" wrapper.

Consider interaction with target.queue - might be a good idea to skip increasing retries if delivery fails due to timeout waiting on limits.

[mughees-urrehman]

Please assign this to me. I would like to work on it

[foxcpp]

Done)