Rework HealthIndicator support

This PR refactors the `HealthIndicator` support in Spring Boot's `Actuator` in the following ways:

1. No multiple instances of `HealthIndicator` can be passed to `HealthEndpoint`
2. A user can simply add a bean of type `HealthIndicator` to the `ApplicationContext` which will then be picked up and used along-side the built-in ones by `HealthEndpoint`
3. Add `HealthIndicator` implementations for Redis and MongoDB
4. Rename `SimpleHealthIndicator` to `SimpleDataSourceHealthIndicator` to make it more obvious what is being tested for health.

refers to #816 and #817