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
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