SASL: Add support for lazy-loaded properties and event callbacks #82
Description
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.