Add mode related switches to `cq.Assembly.export`

[adam-urbanczyk]

          Interesting.  An idea I had was to possibly support this with the `export` method:

assy.export("out_meta.step", mode="meta")
assy.export("out_meta_no_pcurves.step", mode="meta", write_pcurves=False)

Originally posted by @lorenzncode in #1782 (review)

[adam-urbanczyk]

It might also be a per option swtich, i.e. ..., meta: bool=True, ... to support orthogonal options. E.g. flattening and metadata are two such things.