feat: Auto-allocate stack in runtime

[marcbrevoort-cyberhive]

McEliece keys, in particular, take up a large amount of stack space.

Not only do we currently have to configure this manually for builds for each individual platform,
but since the stack space is set in compile time, this RAM will be held on to indefinitely,
even while no keys are being generated.

This pull request solves both issues by running the KEM key generation in a separate thread.
This allows the stack to be created in runtime, which has two benefits:

1. Compile time configuration is no longer required and
2. Rather than taking up memory for the entire duration of the runtime, it is only in use when needed.

Running as before is still supported through a feature flag.