feat: add ipython autocomplete support for MetaflowObject instances

[bsridatta]

relates to #1114

The autocomplete for objects works with this simple addition of returning children ids. Suggestions when initiating classes like Flow(" could be helpful too but I couldn't find a way to do this. But for the class instances of Flow, Run, Step, Task, the autocomplete works and for reference the options are printed by calling the function that ipython would use.

Any thoughts @romain-intel ?

from metaflow import Flow, Metaflow

Metaflow().flows
>>> [Flow('HelloFlow'), Flow('MovieStatsFlow')]

flow = Flow('HelloFlow') # No autocomplete here
flow._ipython_key_completions_()
>>> 
['1680815181013681',
 '1680815178214737',
 '1680432265121345',
 '1680430310127401']

run = flow["1680815178214737"]
run._ipython_key_completions_()
>>> ['end', 'hello', 'start']

step = run["hello"]
step._ipython_key_completions_()
>>> ['2']

task = step["2"]
task._ipython_key_completions_()
>>> ['name']

data = task["name"]
data._ipython_key_completions_()
>>> Error (since no childern), so autcomplete just list unrelated things

[savingoyal]

lgtm! @romain-intel any thoughts here?

[romain-intel]

This looks great. I did do some research about adding in constructor but yes, it's not straightforward so we can merge this. Thanks for the contribution.