[SPEC] Dstack VMM startup handling in manager

[Serial-ATA]

Need to determine whether the Dstack VMM will started:

• By the operator, before the blueprint manager
• By the blueprint manager, before listening for events

If started by the operator, the manager will need some way to do a health check to make sure that it's alive.

If started by the blueprint manager, then it will need to accept the vmm.toml from the operator on the CLI (see https://github.com/Dstack-TEE/dstack/tree/04de4e422bb06f075b4215b2cfc410f5d7ac7aed?tab=readme-ov-file#3-run-components). The CLI will also either need to accept the path to the VMM binary, or there will need to be some installation guide for operators, as none of the Dstack components are installed system-wide, at least traditionally.