Support anonymous read-only access to a bucket

Currently in a secure NiFi Registry, all users must authenticate with a valid identity based on one of the authentication mechanisms that has been configured (client certs, LDAP, or kerberos). If a request comes in with no user credentials, then we create an anonymous user which will be denied access.

We should support a way to indicate that a bucket can be read by anyone (i.e. the anonymous user). This would allow there to be a bucket where publicly available flows can be shared, but only those users with write permissions to the bucket can add flows, and other buckets can remain private.