[Key Vault] Generate nonce for key release by default

[mccoyp]

Per this comment, we may want to generate a (non-cryptographic) nonce when one isn't provided as a parameter to key release. This may or may not be necessary since the parameter is currently optional for the operation and is "just an identifier the client can send in order to detect freshness in certain more complex confidential compute scenarios".

• Investigate whether or not default generation is desirable
• Implement default generation if so

[mccoyp]

After asking the service team about this, generating a nonce won't be necessary.