Add TypedDict type hints for AirflowPlugin list fields

[sivavarma-x]

Most AirflowPlugin list fields (external_views, react_apps, menu_links, appbuilder_views, fastapi_apps, ...) are currently typed as list[Any], giving plugin authors no static signal about the shape of the entries they're expected to supply.
This PR introduces TypedDicts in the shared plugins_manager that capture those contracts and swaps the
list[Any] annotations on AirflowPlugin over to them.

Rolling in one field per commit so each contract can be reviewed on its own.
First up: ExternalViewDict for external_views. Follow-ups for the remaining fields will land in subsequent commits on this PR.

Opening as draft while the rest lands.

• closes: More thorough type hinting for AirflowPlugin attributes #62222

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Was generative AI tooling used to co-author this PR?

• Yes

Generated-by: gpt-5.5 following the guidelines

[boring-cyborg]

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