Part of BSIP48: Add option to update an asset's precision

[abitmore]

Currently precision of asset can't be updated, it's annoying when a user accidentally set an incorrect precision when creating the asset.

This BSIP propose a protocol change so that precision can be updated when there is zero supply.

To implement, can add a field in extensions of asset_update_operation.

[xiao93]

support, thank abit.

[abitmore]

This will be addressed in BSIP48 (#216). Closing.