refactor(api): Refactor VM-invoking API server methods to be less VM-specific

[slowli]

What ❔

• Refactors VM-invoking methods in the API server (e.g., gas estimation) to use less VM-specific logic.
• Simplifies some VM interfaces along the way. E.g., unifies batch and oneshot VM executor results.

Why ❔

Should somewhat simplify ZK OS integration into the API server.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.