Lazy POC to help with updating singleton components in composite commands or middleware usage

[wbreza]

Introduces a Lazy[T] type that we can use to inject into actions that are used within composite actions.

Examples:

• AzdContext (created in init, used in provision & deploy)
• Project config (event handlers registered in middleware, used in many other actions)

[weikanglim]

How about, instead we make Lazy accept a mutation, SetInitializer. This will error if someone has already called GetValue that resulted in an initialization. That way, we can avoid having pointers to lazy. It also pushes the error correctly to where we incorrectly set it, versus undefined behavior.

We can also make it the usage of the wrapped lazy value thread-safe, since we don't expose SetValue (unless needed for tests).