Sets the private key for a server certificate or client certificate.
A secure connection using this SecurityContext will use this key with
the server or client certificate to sign and decrypt messages.
file is the path to a PEM or PKCS12 file containing an encrypted
private key, encrypted with password. Assuming it is well-formatted, all
other contents of file are ignored. An unencrypted file can be used,
but this is not usual.
NB: This function calls ReadFileAsBytesSync, and will block on file IO.
Prefer using usePrivateKeyBytes.
iOS note: Only PKCS12 data is supported. It should contain both the private key and the certificate chain. On iOS one call to usePrivateKey with this data is used instead of two calls to useCertificateChain and usePrivateKey.
Source
void usePrivateKey(String file, {String password});