Add ability for launcher stages to report diagnostics

[Liam Farrelly (lfarrel6)]

Why

Stacked on #337

In order to make the launcher chain more useful than an abstraction on critical workflows, this PR extends the interface to give us diagnostics support. This should unlock the reporting of non-fatal errors and graceful degradations that may occur within the enclave launch process.

Each diagnostic is emitted explicitly by a stage of the launcher. There must be care taken to ensure that the diagnostics only reveal a minimal amount of safe data.

How

• Add a diagnostic callback to the boot context object passed into each stage. This allows the stage to produce its own diagnostic reports
• The diagnostic callback is then extended to the observer which receives all lifecycle events of the launcher chain. The observer can then direct this context as appropriate.
• Evolved the control plane -> data plane healthchecks to include a new diagnostics property which allows arbitrary diagnostics to be reported to the host.