ENH: add an IOC that builds itself from a json file

[tacaswell]

Example input:

{"a:b": ["float", "ai"],
 "c:d": ["float", "ao"]}

> python caproto/ioc_examples/bucket_of_pvs.py --list-pvs --json /tmp/pv_bucket.json 

attn @RussBerg

[danielballan]

The expected structure of the JSON should be documented somewhere. (Currently there is just an example in the PR conversation.) The type_map should be expanded, or the unsupported types should fail more informatively.

[klauer]

A bit too basic to be considered generally useful, and also not as well documented as it needs to be (echoing @danielballan's review)

[tacaswell]

Oh, I was looking for this example when I started https://github.com/NSLS-II/archiver-proxy !