[PLACEHOLDER] Define an API to do various chain-specific validation

[GuillaumeRx]

The new multichain send flow will need to do request validation of various things chain-specific to the snap. Create an API that allows the client to request these validations from the snap.

(The new networks team is experimenting a temporary API through onClientRequest in the interest of time. They will come back with a SIP once they have something "stable" enough).