Refactor metadata to use dataclass hierarchy

[dwikler]

Title:
Refactor SpecModel metadata to use a dataclass hierarchy (BaseMetadata, TableMetadata)

Description:

• Introduce dataclasses for metadata.
• Refactor all code that creates, sets, or accesses metadata to use these dataclasses instead of anytree.Node.
• Update DOMTableSpecParser.parse_metadata and all model constructors to use the new dataclasses.
• Update all usages and tests accordingly.