Add cache specialized for access tokens

[matheuscscp]

Introduce a cache that is specialized for access tokens. This is a thin abstraction on top of the LRU cache adding requirements that are important for managing the lifetime of tokens:

• We consider tokens expired after 80% of their lifetime. This is what kubelet does for rotating ServiceAccount tokens inside pods (ref).
• We need a pessimistic approach for considering tokens expired. With this in mind we store two time points: the usual time.Now().Add(duration), which has a monotonic clock, and also a Unix time point wihtout a monotonic clock. If either report t.Before(time.Now()) == true, the token is considered expired. This is important because Flux may be running in local machines, which are often put to sleep. When they come back up, the monotonic clock will resume from where it stopped, i.e. for the monotonic clock the machine sleep time is considered a "pause". This would cause tokens to be kept beyond their actual expiration, so we use both clocks to ensure we don't return expired tokens on TokenCache.Get().

[stefanprodan]

LGTM

Thanks @matheuscscp 🏅