sourcify: collect proxy information

[elopez]

No description provided.

[dguido]

Code Review Note

Breaking Change: This PR renames implementation_address → implementation_addresses and changes the type from str | None to set[str].

Downstream consumers (Slither, Echidna, Manticore) using compilation_unit.implementation_address will need to update to the new API.

Consider adding a deprecated compatibility property if backward compatibility is needed:

@property
def implementation_address(self) -> str | None:
    """Deprecated: Use implementation_addresses instead."""
    return next(iter(self._implementation_addresses), None)

Otherwise, just ensure downstream tools are updated accordingly.