feat: Add new containerProbe type exec for command execution

[astromechza]

This fixes #113 to add the exec type container probe option. This is equivalent to:

• Compose spec: https://docs.docker.com/reference/compose-file/services/#healthcheck
• Kubernetes pod readinessProbe: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-readiness-probes (see exec)

Most other container runtimes (fly.io, gcp cloud run, ECS app, azure container app) all use http/grpc/tcp health probes instead, presumably because these are cheaper for the dataplane to execute reliably. However since the reference implementations of Score support exec probes, we can add this as an option.

I think it's important we allow both types to be specified for a probe so that the runtime can pick the one it supports and still get the right final behavior.

Examples:

score-compose can convert:

readinessProbe:
  exec:
    - /bin/bash
    - -c
    - "true"

to

healthcheck: ["CMD", "/bin/bash", "-c", "true"]

score-k8s can convert:

readinessProbe:
  httpGet:
    port: 8080
    path: /
  exec:
    command: ["/bin/curl", "-f", "http://localhost:8080"]

into:

readinessProbe:
  httpGet:
    port: 8080
    path: /

Since the http variant of the probe is supported and cheaper to run.

[chrishumanitec]

One open question: is choice of which probe to use if both supported implementation specific?

[chrishumanitec]

Should the spec define which one takes precedence if both are supported - or should that be implementation specific?

[astromechza]

@chrishumanitec I think perhaps if both are supported we should use the command execution, as this likely has deeper or more specific functionality than the simple http one.

[astromechza]

Updated the description.

[mathieu-benoit]

LGTM