[Enhancement] Add the ability to pass initial variables into X-Stepper-Execute-Before sequences.

[CoreyD97]

As the title suggests.

Useful in scenarios where you want to bruteforce a sequence, but the injection point is within the sequence itself.

[er4z0r]

Maybe this could be done by adding another header like:

X-Stepper-Execute-Before-Vars: VAR1="value1"; VAR2="value2" ...