feat: DH-21757: WidgetPlugin support in deephaven.ui #4232

Job	Run time
changes	5s
filter-release	0s
release	0s
test-python (ui) / test (3.9)	55s
test-python (ui) / test (3.11)	1m 3s
test-python (ui) / test (3.10)	55s
build-docs (ui) / check-make-docs	5s
test-python (ui) / test (3.13)	52s
test-python (ui) / test (3.12)	1m 5s
test-js / unit	2m 26s
build-docs (ui) / build-plugin / build-python-package	12s
build-docs (ui) / build-docs	1m 15s
test-results	3s
	8m 56s

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