use `RNP_LOAD_SAVE_PERMISSIVE` flag to parse keys with unknown subkey algorithm IDs

In https://gitlab.com/sequoia-pgp/rnp-sop/-/blob/main/src/lib.rs, the _load_keys() function calls the rnp_import_keys() RNP API. This can be called with the RNP_LOAD_SAVE_PERMISSIVE flag. From the RNP API function description:

If RNP_LOAD_SAVE_PERMISSIVE is specified then import process will skip unrecognized or bad keys/signatures instead of failing the whole operation.

Doing so would probably make the test suite pass the OpenPGP interoperability test suite Mock PQ subkey test. So, in principle, the library supports this behaviour, but as a general-purpose OpenPGP library it leaves the specific behavior up to the caller.