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

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).

Start by reading the new multichain send flow and the temporary API exposed through onClientRequest. Determine the chain-specific validations the client must request from the snap and define the stable API boundary; done means the API supports those validation requests without relying on the temporary experiment.

Written by the indexing model from the issue text.