A NetworkManager client requires an API to validate and decode
a base64 secret -- like it is used by WireGuard. If we don't have
this as part of the API, it's inconvenient. Expose it.

Rename it from _nm_utils_wireguard_decode_key(), to give it a more
general name.

Also, rename _nm_utils_wireguard_normalize_key() to
nm_utils_base64secret_normalize(). But this one we keep as internal
API. The user will care more about validating and decoding the base64
key. To convert the key back to base64, we don't need a public API in
libnm.

This is another ABI change since 1.16-rc1.

(cherry picked from commit e46ba018)