Allowing --variable read a portion of provided files, makes curl work on
partial files for any options that accepts strings. Like --data and others.
The byte offset is provided within brackets, with a semicolon separator
like: --variable name@file;[100-200]"
Inspired by #14479
Assisted-by: Manuel Einfalt
Test 784 - 789. Documentation update provided.
Closes#15739
