Create generally useful disk health checks

[natalie-zamani]

There are two health checks I have in mind for this:

• A disk space health check, that performs a check to determine if there is some configurable level of available disk space on the host the app is deployed to. This should be opt-in, if an app depends on some disk usage behavior. For inspiration: https://github.com/oracle/helidon/blob/master/health/health-checks/src/main/java/io/helidon/health/checks/DiskSpaceHealthCheck.java
• One that performs a write against the file system, to ensure that there isn't a read-only file system issue.