Add v2 accessors.

[danielballan]

The use case is to write functions that work on old-style databroker.Header and new-style databroker.BlueskyRun:

In [3]: def f(header):
   ...:     "Accept databroker-style or tiled-style Header/BlueskyRun."
   ...:     run = header.v2  # works if header is already a tiled BlueskyRun or if it is a databroker Header
   ...:     return run['primary']['data']['fccd_image']
   ...: 

In [4]: from tiled.client import from_profile

In [5]: c = from_profile("csx")
OBJECT CACHE: Will use up to 20_214_985_728 bytes (15% of total physical RAM)

In [6]: run = c.values().last()

In [7]: f(run)
Out[7]: <ArrayClient shape=(5, 1, 1000, 960) chunks=((5,), (1,), (1000,), (960,)) dtype=int16 dims=('time', 'dim_0', 'dim_1', 'dim_2')>

In [8]: from databroker import Broker

In [9]: db = Broker.named("csx")
OBJECT CACHE: Will use up to 20_214_985_728 bytes (15% of total physical RAM)

In [10]: f(db[-1])
Out[10]: <ArrayClient shape=(5, 1, 1000, 960) chunks=((5,), (1,), (1000,), (960,)) dtype=int16 dims=('time', 'dim_0', 'dim_1', 'dim_2')>

[gwbischof]

Looks good!