* NIFI-8363 Added Single User Login Identity Provider and Authorizer
- Reads and writes username and hashed password in login-identity-providers.xml
- Generates random username using java.util.UUID.randomUUID()
- Generates random password using java.security.SecureRandom with Base64 encoding
- Writes generated password hash using bcrypt
* NIFI-8363 Updated SingleUserAuthorizer to require SingleUserLoginIdentityProvider
* NIFI-8363 Added handling of null login identity provider property