integrate v3 onion service key generation into tor

Currently, a separate openssl command is needed to generate the keys used for v3 onion service client authentication. This makes it difficult for a mobile application, which generally doesn't have the ability to run openssl commands in a shell, to enable client authentication on v3 onions it is hosting.

This is specifically relevant to work on Onion Share mobile apps for iOS and Android.

Please let me know what options we might have in C, Rust or Go, to more generate these keys, or if it is likely/possible to integrate this step into tor itself, as it was with v2 keys.