Provide a way to implement a HealthChecker with peioric polling easily.

[trustin]

A user wants to implement a HealthChecker that updates its healthiness by polling an external source (e.g. database) periodically, so that HealthChecker.isHealthy() merely returns a cached value. This is mainly because:

• HealthChecker.isHealthy() that takes long time can make a server look unhealthy.
• Each health check request triggers HealthChecker.isHealthy(), and thus it is suboptimal to actually check the health of the external source.

It'd be nice if we provide a utility that simplifies the implementation of this pattern, e.g.

HealthChecker.ofFixedRate(Supplier<CompletionStage<Boolean>> healthChecker, Duration interval, double jitter)
HealthChecker.ofFixedDelay(Supplier<CompletionStage<Boolean>> healthChecker, Duration delay, double jitter)

Discussion at Slack: https://line-armeria.slack.com/archives/C1NGPBUH2/p1615691261133800

[trustin]

Alternatively, we could keep the timestamp of last healthChecker.get() and return the cached result only when the recorded timestamp is old enough. This is simpler to manage because we don't have to schedule a periodic task using an event loop.