Support loader-specific args in iris load functions.

[pp-mo]

This has been requested, to support #3647.
Previously considered for other cases, e.g. #3333 (but we didn't proceed with that).

Note: decided not to implement arbtrary additional (unnamed) args here, as the already optional positional args (constraints, callback) would make that rather messy.
We could still implement **kwargs instead of a specific keyword, if preferred? But I thought that might also lead to confusion..

Note: the changes reach further than I had imagined (into iris.fileformats and iris.io), but I believe this still does not introduce any breaking changes. Nevertheless, I've marked it up for Iris 3.0

[pp-mo]

I'm reviving this because I am again contemplating #3333 following some further experience with problem cases.
It will be needed to implement that.

Unfortunately we currently have a train wreck in the CI due to a problem with latest-version matplotlib docs.
So will come back here when that is fixed

[pp-mo]

#4572 now proposes an alternative control mechanism.
I'm thinking this is actually superior to load kwargs, because it groups control features in separate areas?

[pp-mo]

Not currently needed / no driver
We can re-open if needed

plus ... much of this (like chunking control) could be done with context managers instead ?
-- like the unstructured load