Asynchronous APIs

Note: async is overloaded. Usage here refers to an "asynchronous endpoint", as described in RFD 4, not the async Rust keyword.

As discussed in https://github.com/oxidecomputer/omicron/pull/385#issuecomment-969200604 , Omicron is growing a reasonably sized API surface. As @davepacheco mentioned, "most things that use sagas... probably ought to be async".

At the time of writing this bug, none of our endpoints are async. We would benefit from:

• Identifying long-running operations, and defining a set which are good candidates to be async (at minimum, "create instance" seems like a good candidate).
• Prototype a couple endpoints as async, returning an ID, and providing a wait-for-completion API.
• Iterate on ergonomics, providing utilities (or possibly changes to Dropshot) making these operations as easy to write as possible.

Start with RFD 4 and the discussion linked from pull request 385 to understand asynchronous endpoint semantics. Identify long-running operations, beginning with create instance, then review how a couple of endpoints could return an ID and expose wait-for-completion APIs. Done means a defined candidate set, prototypes, and an agreed ergonomic direction, including whether Dropshot needs changes.