Redesign transaction proposing, authorizing and sending logic

[willemolding]

Currently a unified full key (in seed phrase form) must be passed to webzjs in order to authorize a transaction.

In the near future we want to be able to request a proposal from WebZjs which returns a partially formed payload (something like a partially signed bitcoin transaction) to be passed to an Account Provider (e.g. snap) for approval and signing.

This issue is to change the API to be able to support this use-case in the future, while also allowing a less secure in-browser signing in the shorter term