Implement useAbstractionForFiles for webclient library

[iyzana]

This pull-request extends #6912 by providing an implementation of the new useAbstractionForFiles flag for the webclient library.
The original issue was #6715 . I left a comment on that ticket mentioning that the functionality would be nice to have for the webclient as well.
In this PR I mostly just adapted the changes that were necessary for the resttemplate to work for the webclient.

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[iyzana]

I'm not sure why the circleci build is still failing

[wing328]

@iyzana can you please resolve the merge conflicts when you've time? We'll look into the CI issue later if it's still failing.

[iyzana]

rebased

[wing328]

Pushed f9a8614 to fix CI failures. Let's see how that goes.

[MosheElisha]

@noako when you update the java.md docs as part of #8549 , please consider mentioning that the useAbstractionForFiles is also supported for webclient library.