Allow defining HealthChecks without CDI

The current design around MP Health enables deployments to specify HealthChecks using CDI. Some deployments (like Keycloak) doesn't use CDI at all. We would need a way to inject additional HealthCheck instances into Wildfly without enabling it.

From the implementation perspective, it would be useful to have some sort of static accessor (or maybe something defined in jboss-structure.xml) that would enable use to inject some custom HealthCheck.

References:

• Keycloak Observability Design
• * Keycloak Healthcheck Design