feat: add virtual arrays

[pfackeldey]

marked as draft because:

• how should the API look like?
• need to wait for feat: implement virtual arrays awkward#3364

If you want to try it with the corresponding awkward branch:

from coffea.nanoevents import NanoEventsFactory

events = NanoEventsFactory.from_root({"tests/samples/nano_dy.root": "Events"}, mode="virtual").events()

events.Muon.pt
# <Array [??, ??, ??, ??, ..., ??, ??, ??, ??] type='40 * var * float32[param...'>

events.Jet.eta
# <Array [??, ??, ??, ??, ..., ??, ??, ??, ??] type='40 * var * float32[param...'>

events.run
# <Array [??, ??, ??, ??, ..., ??, ??, ??, ??] type='40 * uint32[parameters={...'>

import awkward as ak

events.Muon.pt.layout
# <ListOffsetArray len='40'>
#     <offsets><Index dtype='int64' len='41'>[## ... ##]</Index></offsets>
#     <content><NumpyArray dtype='float32' len='##'>
#         <parameter name='__doc__'>'pt'</parameter>
#         [## ... ##]
#     </NumpyArray></content>
# </ListOffsetArray>

ak.materialize(events.Muon.pt).layout
# <ListOffsetArray len='40'>
#     <offsets><Index dtype='int64' len='41'>
#         [ 0  0  0  0  0  2  3  5  5  5  5  6  6  6  6  7  7  7  7  7  7  7  7
#           7  9 11 11 13 13 15 15 16 17 17 17 18 18 18 18 18 18]
#     </Index></offsets>
#     <content><NumpyArray dtype='float32' len='18'>
#         <parameter name='__doc__'>'pt'</parameter>
#         [76.75332   20.131409  31.038704  50.641342  14.330103  16.724983
#          13.908063  46.498886  40.667812  51.43576   39.59279   38.89543
#          33.712822  17.082792  14.526995   4.3605423 10.117709  17.949919 ]
#     </NumpyArray></content>
# </ListOffsetArray>

[pfackeldey]

I've deprecated delayed and added a new argument called mode. mode can be "eager" (loading all columns), "virtual" (creating a virtual array that loads on demand), or "dask" which creates a dask-awkward array. If mode is None (default), delayed=True will correspond to the "dask" case, and delayed=False to the "virtual" case. If mode is given, it will take precedence over delayed. I added a deprecation for delayed with an unknown deprecation date.

The only thing missing now is a new awkward version that contains virtual arrays. Otherwise, this PR is ready.