Disable 2mb limit for custom incrementalCacheHandler (#59976)
### Fixing a bug
### What?
Disable 2MB limit for custom incrementalCacheHandler
### Why?
The limit is necessary because `FetchCache` has a 2MB limit, but it
seems there was a miscommunication regarding the key coincidence, where
`fetchCache` is a flag indicating that the method is called from fetch,
rather than indicating that the `FetchCache` Provider is currently being
used.
We do not use Vercel, and as I understand it, we do not have the
opportunity to use this functionality.
In any case, it is more important for us to increase the limits, and in
some cases, using a file storage is even preferable.
### How?
I have created a flag that determines whether the use of `FetchCache` is
possible at least in theory - if no custom provider is passed, and
additionally configured it so that it is not an implementation of
`FetchCache` as a protection against special individuals (*like me :)*).
If everything is fine, I will write proper tests.
Also, I would like to recommend making `FileSystemCache` public (_i.e.
support it as public functionality_) so that it can be imported and
extended or simply used to fix only it.
Fixes #48324 (partially)
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
