Extract blob size checking logic from push rule

What does this MR do and why?

Extract blob size checking logic from push rule

This change is a refactor to prepare for the use of file size checks outside of push rules (upcoming changes: !125245 (merged))

Contributes to: #393535 (closed)

How to setup and validate locally

1. Create a new project
2. Settings -> Repository -> Expand Push rules -> Set Maximum file size (MB) to 1 MB
3. Try to push a file that is smaller than 1 MB. See that the push is allowed. (e.g head -c 1048576 /dev/urandom > borderline)
4. Try to push a file that is larger than 1 MB see that the push is not allowed. (e.g head -c 1048577 /dev/urandom > oversized)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #393535 (closed)