Run a Wasm file as an application without a manifest

[itowlson]

Fixes #2477. Or at least it explores what a solution might look like - there are definitely other paths to consider.

For example, this goes the route of constructing an in-memory spin.toml, because the route from spin.toml to locked app to trigger is well-exercised. But we could bypass that and construct a lockfile directly - a lot of spin.toml concerns are not (yet) relevant to manifestless applications.

Or we could start planning more extensively for Future Things - this POC doesn't really provide any support in terms of the OCI proposal.

One thing we discussed internally was whether to put this on spin up or on a new command such as spin serve (by analogy with wasmtime serve). For the sake of this POC I put it on spin up and inferred "bare file or manifest" based on the file extension. This is not intended as a proposal, just a thing to dangle the actual loader code off.

Anyway hopefully provides us with something to argue about grin