Add testnet/mainnet type for assets

[zakarialounes]

When retrieving all assets from the @chain-registry package, it is not possible to differentiate between mainnet/testnet assets, only for the chain is it possible. A boolean should be added if possible. What do you think about it? Thanks in advance.

[JeremyParish69]

This should be handled by the package looking at which folder the chain/asset data comes from (testnets vs top level).