Spike: Support decimal conversion in internal bridge

[rhuairahrighairidh]

Description

This allows assets connected in the internal bridge to support different decimal precision on each side. eg 8 decimal places on the sdk side and 18 on the evm

When converting down the way (eg an 18 decimal number to an 8) the transfer amount must be divisible to leave no dust. Not the best user experience, so we could alternatively return the dust to their account.

Only supports the evm native asset bridge.

Checklist

• [ ] Changelog has been updated as necessary.