Add support for local kernel execution

[agoose77]

Proposal

Right now, myst has no built-time execution support (e.g. to generate figures). We are in a strong position to implement cache-aware builds, and I don't think it will be too much work given the fact that we're just re-using the Jupyter APIs.

c.f. #330

Additional notes

• We may want a myst execute command to avoid building any render targets. Maybe just myst build?
• Add support for annotating documents as "un-cacheable" e.g. things that depend upon random inputs.
• Multiple cache backends? In future, could perhaps centralise a cache among users for e.g. a JupyterHub deployment that controls the execution environment

[agoose77]

This is closed by #873