Readiness health checks story 4 manifest endpoints

[mariash]

This PR is part of a larger effort detailed here in this Cloud Controller PR and here in this CFF RFC.

In this story

• readiness health check properties are returned by /v3/apps/:guid/manifest endpoint.
• documentation is updated for /v3/apps/:guid/manifest endpoint to include example of readiness health check properties.
• cf create-app-manifest returns readiness health check properties in the generated manifest as a result of the above change
• whenever readiness health check properties are changed cf push properly displays the diff for updates. /v3/spaces/:space_guid/manifest_diff was changed to return diff for readiness health check properties.

---
applications:
- name: my-app
  stack: cflinuxfs4
  services:
  - my-service
  routes:
  - route: my-app.example.com
    protocol: http1
  processes:
  - type: web
    instances: 2
    memory: 512M
    log-rate-limit-per-second: 1KB
    disk_quota: 1024M
    health-check-type: http
    health-check-http-endpoint: /healthy
    health-check-invocation-timeout: 10             # 👈 new property
    readiness-health-check-type: http               # 👈 new property
    readiness-health-check-http-endpoint: /ready    # 👈 new property
    readiness-health-check-invocation-timeout: 20   # 👈 new property

Please note: health-check-invocation-timeout property was not exposed before. Please let us know if this was done intentionally. It was added to be consistent with readiness-health-check-invocation-timeout.

Acceptance Setup

1. Deploy capi-release with this PR.
2. Push an app without readiness health check properties
   1. cf curl /v3/apps/:guid/manifest should return readiness-health-check-type: process
   2. cf create-app-manifest app-name should generate manifest with readiness-health-check-type: process
3. Push an app with readiness health check properties in the manifest for the app:

   readiness-health-check-http-endpoint: /ready
   readiness-health-check-invocation-timeout: 2
   readiness-health-check-type: http
   

   1. cf curl /v3/apps/:guid/manifest should return these properties
   2. cf create-app-manifest app-name should return these properties
4. When changing one of the properties in the manifest and updating an app a proper diff is displayed.