Add appGUID attribute to droplet resource [v8]

[gmllt]

Where this PR should be backported?

• [x] main - all changes should by default start here (PR on the main branch)
• [x] v8
• [ ] v7

Description of the Change

The V3 API allows you to access to the relationships existing beween a droplet and its related app. This PR make this relationship visible trough a new AppGUID attribute in the resource droplet.

The community cf_exporter currently use the /v2/spaces/:guid/summary endpoint to gain access to buildpack associated to an application. Have this relationship visible would help get rid of calls on this endpoint and retrieve buildpacks attached to an application through its droplets.

This feature is necessary for applications using the cli product in go development to established relationship between an application resource and its droplets.

This feature will not cause breaking change.

[gmllt]

PR on main branch : 2932

[a-b]

@dependabot rebase