[12.x] Add encoding validation rule for uploaded files

[ziadoz]

This adds a new encoding validation rule that checks the contents of a file matches a specific encoding using mb_check_encoding(). In my case I want to ensure user uploaded files such as spreadsheets from office suite software is actually UTF-8, so it can be safely processed.

It can be used as a typical validation: ['file', 'encoding:utf-8'] or via the file rule builder: File::rule()->encoding('utf-8'). It also works for strings.

[shaedrich]

Would it make sense to have a strict (or loose) flag to allow subsets/supersets, e.g. ascii would allow for utf-8 files as they are ASCII-compliant

[ziadoz]

@shaedrich I was trying to keep things as simple as possible here.

[taylorotwell]

Is this rule actually specific to files or is it just for any string?

[ziadoz]

@taylorotwell It works for both files and strings.

[taylorotwell]

Thanks