[Upgrade Assistant] Decouple ML snapshots routes schema from saved objects attributes

[yuliacech]

In preparation for versioned HTTP APIs, we need to make our endpoints schemas explicit and decouple them from underlying saved objects types. In Upgrade Assistant, the routes for ML snapshots implicitly rely on saved objects attributes (see this file).
We need to create separate TypeScript interfaces for the endpoints schemas and for the saved objects attributes (if none exist already). A guide for this process can be found here and an example in this PR.

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)