chore: Implement LazyValue helper

[dazuma]

This helper is the core piece of the updated implementation. It provides:

• Lazy computation/evaluation of data. The object is constructed with a block that specifies how to compute the value (e.g. make a request to the metadata server), but it isn't actually executed until the first time the value is accessed.
• Caching/memoization so a value gets computed only once, and the cached result is returned on subsequent accesses.
• Expiration of that memoization, for information such as access tokens.
• Retry, tailored especially for metadata server scenarios (e.g. a small number of retries to handle network instability, and a warmup period of retries to handle the initialization case).
• Robust thread safety, assuring in particular that, modulo retries and expiration, at most one thread will ever do the computation/lookup for any particular value.
• Both individual value and key-value interfaces.

Some of this functionality is similar to that provided by Concurrent::Delay and Concurrent::LazyRegister, but nothing in concurrent-ruby provides everything we need.

[bajajneha27]

What's the default expiration that we're setting for cache expiry?

[dazuma]

By default, memoized values never expire. I'll make that more explicit in the documentation.

[bajajneha27]

Just curious why we're not using retry gem for retrying.

[dazuma]

Because it's not a simple retry, but one whose conditions and delays are dynamic and tied closely to information from the state of the LazyValue. For example, to determine the delay until the next retry, we need to get the wait time expiration, that is, the next time the computation engine will allow another computation attempt to start.