feat(vector): default API bind and readiness for chart-managed config

[kurochan]

This PR updates the vector chart defaults to make health checks easier to use with chart-managed configuration.

• Changes default generated Vector config api.address from 127.0.0.1:8686 to 0.0.0.0:8686
• Adds a default readinessProbe only for chart-managed config (no customConfig / no existingConfigMaps)
  • httpGet.path: /health
  • httpGet.port: api
• Keeps livenessProbe opt-in (no default liveness behavior change)
• Updates README and values comments with API exposure and probe behavior notes

Considerations

• This is a behavioral change for users relying on chart defaults: API bind changes from loopback-only to pod-reachable (0.0.0.0).
• Security impact: wider API reachability means access should be restricted with NetworkPolicy and careful Service/Ingress exposure (this PR intentionally does not add default NetworkPolicy resources).