Implement automatic DSDL installation from the central repository

[pavel-kirienko]

See the description here: https://forum.opencyphal.org/t/central-repository-of-dsdl-namespaces/1817

By default, the auto-compilation should be disabled. PyCyphal should provide a function like pycyphal.dsdl.enable_automatic_dsdl_fetching(repository_uri: str = 'https://dsdl.opencyphal.org/repository.yaml') -> None, which should be invoked during early initialization. If invoked, it would modify the autocompilation hook to look for the missing namespaces in the registry and fetch them if necessary.