importccl: refactor import processor to use ProcessorBase

[pbardea]

This commit refactors the import processor to use ProcessorBase.
Although this processor doesn't have any inputs and the main utility
that ProcessorBase provides is management of draining inputs, there is
still some utility for this processor to embed a ProcessorBase. As
tracing and other infrastructural improvements are made to
ProcessorBase, this allows the import processor to stay up to date with
these changes.

The way that this processor works is that it kicks off a goroutine on
start that will begin the import and start sending progress updates over
an internally maintained channel. Next() will continually read from that
channel until it is fully consumed, and then read the returned summary
generated by the goroutine.

Import is build this way so that it can manage concurrency internally to
the processor.

Relates to #57293.

Release note: None

[cockroach-teamcity]

This change is 

[pbardea]

The changes to the other processors are very similar, I can add them onto this PR as other commits but feedback on the general approach would be appreciated.

I think since there is so much similarity between the approaches, it makes sense to make a general async processor, but thought it would be easier to review if I first make the changes to all of the processors and then factor out the common logic into a common processor. Thoughts appreciated!

[pbardea]

TFTR
bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)