(API) Cluster Health report unassigned_primary_shards (#111727)

[lukewhiting]

This PR adds a count of currently unassigned primary shards to both the /_cat/health and /_cluster/health endpoints. This is to aid cluster administrators in estimating the time remaining for a cluster to go from RED to YELLOW status as per enchancement request #111727.

Tests and doc updates are in place with this PR and manual testing with ./gradlew run has been conducted on the endpoints to ensure correct output.

Known Limitations

• Testing
  • Due to limitations in the YAML REST test framework skip functionality, YAML REST tests for this endpoint are disabled when running a mixed version cluster by using a cluster version number synthetic feature to skip when any member of the cluster is not at a version greater than when this change is due to be introduced