* Modularize shard availability service
This commit moves the `ShardsAvailabilityHealthIndicatorService` to a package and modularizes it
with exports so that Serverless can make use of it as a superclass.
Relates to #101394
* Make the ShardsAvailabilityIndicator pluggable
This commit makes the `ShardsAvailabilityIndicator` pluggable. It currently provides the existing `ShardsAvailabilityHealthIndicatorService` (without moving it) for the `shards_availability` output in the health report API.
This will allow us in the future to provide a separate `shards_availability` plugin to override the way that health is reported for shards.
