[C++] Provide way for extension array to provide it's own value pretty printer

[wjones127]

Describe the enhancement requested

We are looking at implementing BFloat16 data type as an extension type based on a FSB[2] storage type. In Python we can write the repr method for the array, but ChunkedArray and Table will print the storage type, which looks like nonsense to users:

>>> tab = pa.table({ "x": bfloat16_array([1.1, None, 3.4]) })
>>> tab
pyarrow.Table
x: extension<lance.bfloat16<BFloat16Type>>
----
x: [[8D3F,null,5A40]]
>>> tab['x']
<pyarrow.lib.ChunkedArray object at 0x12a1f3a60>
[
 [
 8D3F,
 null,
 5A40
 ]
]
>>> tab['x'].chunk(0)
<lance.arrow.BFloat16Array object at 0x0000000129818a60>
[1.1015625, None, 3.40625]

lance-format/lance#1002

Component(s)

C++

[thisisnic]

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