Allow any value text to be passed to the stepper so we can control the value

[richardstevens]

What does this PR do? (please provide any background)

This allows us to send a value of our own into the input, this allows us to use the props to correctly increment and decrement but then display the result of this how we would like.

What tests does this PR have?

Usual tests and linting, i have updated the stepper test to include the setting of the valueText prop

How can this be tested?

This cant be tested until the new wizard repo is also updated

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[hxpaul]

Small recommended change but otherwise ok

[elliottcrush]

All good, works a treat.

[hxpaul]

review :one: all good now :+1: