Allow for specifying a unit of the value being transferred.

[deuszx]

Is your feature request related to a problem? Please describe.

Currently, when calling contract's endpoint that is payable, the Value field unit is fixed to the main unit of the chain (think ETH, DOT, etc.) while when working on a contract we're usually working with the basic unit of the chain's currency. polkadtojs.app allows for choosing the unit (milli, micro, Kilo, etc.), it would be good to provide this feature-parity (pun not intended).

Describe the solution you'd like
A dropdown menu where basic units are available.

Describe alternatives you've considered
n/a

Additional context
Current state of affair in Contract UI:

and screenshot from polkadot.js app for comparison