feat: Dataclass serialization support for deephaven UI

[mattrunyon]

Fixes #757

Not really sure how to best test this in Python unit tests given our test setup. I tested manually by creating my own dataclass with this code

import dataclasses

@dataclasses.dataclass
class Databar:
    column: str
    color: list[str] | None = None

from deephaven import time_table, ui

_t = time_table("PT1S").update(["X=ii", "Y=X", "Z=X", "A=X", "B=X", "C=X", "HalfX=X/2", "LogX=X > 0 ? log(X) : 0", "X2=X*2"])

t = ui.table(_t, databars=[
    Databar(column="B"),
])

[mofojed]

@wusteven815 has some changes about to merge as well, just need to make sure his changes in Renderer are captured, in particular his changes to _get_context_key:

    if isinstance(item, Element):
        key = item.key
        return key if key is not None else f"{index_key}-{item.name}"

https://github.com/deephaven/deephaven-plugins/pull/810/files#diff-5e86d7b937ea83974fe4548642726e5c876a1e47aaa9ea54fb246cd43ae192a8

[mofojed]

We should check here before creating child contexts unnecessarily... pretty much if _render_item is going to fall into the last case where it just returns item instead of rendering into a context, we shouldn't bother creating the child context.

[mofojed]

This doesn't really need to be in an else, but paired with my other comment you may want to remove this entirely depending on how you clean it up.

[mofojed]

Should be able to add a unit test in test_renderer.py, and/or e2e tests as well if you want to add some tests for this.