feat: Implement python item_table_source

[jnumainville]

Fixes #414
Adds Python side of item_table_source and required refactoring.

[jnumainville]

@mofojed I'm wondering if it actually makes sense to make this a "Python side only" concept. There's not anything really "UI" related. The dictionary could just be merged into the props on component creation.
Or, at least maybe that makes sense for now so this doesn't break anything but also isn't waiting on JS changes?

[jnumainville]

I went ahead and did the unpacking
It works with this example.
Doesn't have to be a long term solution but at least this makes any fixes we need to make independent of this PR

import deephaven.ui as ui
from deephaven.ui import use_state
from deephaven import time_table
import datetime
 
column_types = time_table("PT2S", start_time=datetime.datetime.now() - datetime.timedelta(seconds=2000)).update([
    "Int=new Integer(i)",
    "Double=new Double(i+i/10)",
])

@ui.component
def picker():
    value, set_value = use_state(1000)

    source = ui.item_table_source(column_types, key_column="Int", label_column="Int")

    # Picker for selecting values
    pick = ui.picker(
        source,
        label="Int",
        on_selection_change=set_value,
        selected_key=value
    )

    return ui.flex(
        pick,
        ui.text(value, key="text1"),
        direction="column",
        margin=10,
        gap=10,
    )

picker_column_types = picker()

[mofojed]

Looks good, we should also add to the examples.

[mofojed]

Looks good to me, will need to merge latest and fix conflicts.

[mofojed]

Suggestion to clean up the example doc a little bit.