server, cloud: plumb an ExternalStorage memory monitor

[adityamaru]

This change adds a dedicated ExternalStorage memory monitor
as a child of the root memory monitor. This can be used to memory
monitor ExternalStorage operations.

As a start, we memory monitor the ChunkSize that every Azure,S3 and GCS
Writer will buffer when uploading file chunks to storage. During a backup
this could be num nodes * ChunkSize of memory which was previously
unmonitored.

Release note: None

[cockroach-teamcity]

This change is 

[adityamaru]

First commit is from #80668.

[adityamaru]

CI failures are just lint fixups, so this is RFAL!

[dt]

Would it make sense to define a common wrapper and change MakeExternalStorage to always return a wrapped specific implementation?

We already have it doing that with a wrapper that is only concerned with limiters, but we could generalize that wrapper so it is responsible for all common tasks, like wrapping returned readers/writers in rate limiters, reserving the common buffer size for any created reader or writer / freeing it on close, etc?

Having every implementation need to remember to do the grow/shink calls just right makes me a little nervous long term.

[adityamaru]

Would it make sense to define a common wrapper and change MakeExternalStorage to always return a wrapped specific implementation?

I think it does, I did briefly think about it in the context of having a common place where we wrap storage-specific errors as transient. @stevendanna is this going to interfere with the accounting change you are trying to check-in?

[stevendanna]

I definitely like the idea of implementing this in a wrapper.

@stevendanna is this going to interfere with the accounting change you are trying to check-in?

I'm not too worried. I think if we structure the code well, the worst case is that my backport will need just a few lines of code removed in a couple of places.

[stevendanna]

Another follow up question here: Currently, we are setting up this monitor at server-startup and passing it into the builder. This is nice since then the caller doesn't have to worry about it. But, do we have cases where it would make more sense to allow the caller to pass in a monitor?