This commit adds `ElasticsearchRestHealthIndicator`, a new
`HealthIndicator` for Elasticsearch, using the Elasticsearch "low level
rest client" provided by the
`"org.elasticsearch.client:elasticsearch-rest-client"` dependency.

Note that Spring Boot will auto-configure both low and high level REST
clients, but since the high level one is using the former, a single
health indicator will cover both cases.

See gh-15211