curl: add byte range support to --variable reading from file

Allowing --variable read a portion of provided files, makes curl work on
partial files for any options that accepts strings.

curl: add byte range support to --variable reading from file

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

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