Teach nitriding to act as a boot loader

[NullHypothesis]

For some use cases, we want the ability to load an enclave application at runtime. This can happen in various ways, like via an HTTP API (e.g., we PUT a new enclave application whenever needed) or via Docker (e.g., nitriding fetches the enclave application from a Docker registry).

Ideally, nitriding should also have the ability to maintain an append-only log that keeps track of the evolution of enclave applications over time. Perhaps we can also take advantage of Sigsum to make this append-only log permanent.