Add Artifact.{annotations,format,storage} setting context manager(s)

[JacobHayes]

This will allow us to more easily set fields for all Artifacts initiated within the block, including those generated by Producers. Ex:

with Graph(...) as g:
    with Artifact.defaults(annotations=[...], format=..., storage=...): # maybe better name than "default"
        g.artifacts.a = A(...)
        g.artifacts.b = compute(a=a)

We can likely use the arti.context to track stacks for each field (allowing nesting contexts). On Artifact initialization (or rather, in a pydantic @validator("...", always=True)s), we'll pull from the stack:

• annotations should concatenate the arti.context annotation stack with those passed explicitly
• format and storage should default to the last item in their respective arti.context stack, if not set explicitly

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Something similar for Statistics and Thresholds would be interesting, but may be a bit trickier (eg: a Statistic may not work for all types, etc).