feat: add ast plugin to read a global literal from a Python file

[henryiii]

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Adds a dynamic_metadata.ast plugin: given input (a Python file), name (a module-level global), and field, it parses the file with ast — never importing it — and returns the literal value via ast.literal_eval. Annotated assignments work, and the last assignment wins, matching execution semantics. Unlike regex, the value keeps its Python shape, so list/table fields can be filled directly (tuples become lists); shape validation reuses _process_dynamic_metadata.

Also extracts the string-settings validation loop shared with regex into _require_str_settings (pylint duplicate-code).