Add custom decimal sizes for custom assets

[luizstacio]

Motivation

When working with custom Assets, having different decimal sizes is typical. Right now, the wallet only supports 9 decimals.

Usage example

A custom token with 3 decimals, or a custom token with a 10 decimals.

Possible implementations

To do;

• Add a decimal field on the Asset on the Create Custom Asset form.
• Add the new decimal format on the inputs and places where we display amounts. If a component does not exist to abstract this, we should create it to avoid formatting it at all places.
• Add tests to cover this on Transfer and TX History.

Ref: https://forum.fuel.network/t/imposible-to-change-decinals-of-asset-in-the-wallet/3084/1