Support encrypted password for ZK cred/ACL providers

The VMParamsSingleSetCredentialsDigestZkCredentialsProvider takes a zkDigestPassword in as a plain-text JVM param, and the VMParamsAllAndReadonlyDigestZkACLProvider takes both zkDigestPassword and zkDigestReadonlyPassword.

Propose to give an option to encrypt these password using the same mechanism as DIH does:

1. Add a new VM param "zkDigestPasswordEncryptionKeyFile" which is a path to a file holding the encryption key
2. Store an encryption key in above mentioned file and restrict access to this file so only Solr user can read it.
3. Encrypt the ZK passwords using the encryption key and provide the encrypted password in place of the plaintext one

We could also create a utility command that takes the magic out of encrypting the pw:

bin/solr util encrypt [-keyfile <file>] <string>