Provides extension HealthCheck for MongoDB, Neo4j and Kafka

[loicmathieu]

Description
Provides health check for some of the existing extensions.
It will not be exhaustive, please advise if some other extensions deserve an health check in the same batch of health check.

Implementation ideas

MongoDB Health Check

Provides a readiness health check for MongoDB.
Proposed implementation: use the MongoClient to list the databases.
Enabled by default.

Neo4J Health Check

Provides a readiness health check for Neo4J.
Proposed implementation: use the Bolt Driver to make a query to Neo4J.
Enabled by default.

Kafka Health Check

Provides a readiness health check for Kafka.
Proposed implementation: use the Kafka AdminClient to connect to the cluster and check it's state.
Disabled by default.

This is my opinion that as Kafka has built in resiliency for both producer and consumer, the health check should not be enabled by default. Some must have good arguments to enabled it by default and I'm open to discussion

[geoand]

@cescoffier might have some comments on this