SASL: Add support for lazy-loaded properties and event callbacks

[nevans]

Lazy loading can be used with:

• password prompts,
• web browser interactions to acquire/refresh OAuth2 access tokens,
• KMS lookups
• audit logs
• dynamic values that can't be computed until data from the server has been seen.

Eagerly doing these actions can be wasteful, frustrating to the user, or violate security policies.