Allow defining custom ABIs/Structs for use in a `transact` call.

[aaroncox]

The way Wharf transact calls currently work might always require loading the ABIs.

Core doesn't require this and allows for pre-defining structs/ABIs to prevent this API call, and Wharf should too.

Investigate what will be required for a session to perform transactions and preload this data, to prevent those calls from being required.