Add out-of-the-box health indicator for reactive MongoDB

[odrotbohm]

This section of the reference documentation suggest that an indicator would be auto-configured in case MongoDB was in use. Upon close inspection it seems that the indicator expects a MongoTemplate which is not available if you use the new reactive APIs. I guess an additional one is needed based on ReactiveMongoOperations.

I guess the same could be done for Cassandra and Redis.

I'd also suggest to refactor the current one to work against MongoOperations instead of the implementation class.

[wilkinsona]

Thanks, @olivergierke. We already have one for Redis but Cassandra does seem to be another oversight. I've opened #11767 for that.

[izeye]

This looks resolved via #11997.

[snicoll]

Indeed @izeye, thanks!

Duplicate of #11997