Support middleware in getValue

[sandhilt]

What would you like to be added:
Allow add an middleware in get, set or reset value.

unform/packages/core/lib/types.ts

Line 21 in 786bbdf

path?: undefined

Why is this needed:
Sometimes, we need convert some value before submit event is triggered, so if change getValue method, we need set setValue and resetValue. It's annoying.

[TriStarGod]

@sandhilt Doesn't registerField already provide this? An example is found at https://unform.dev/quick-start/