Identity: add an applicationcredentials users extension package

[kayrus]

Openstack API ref: https://developer.openstack.org/api-ref/identity/v3/index.html?expanded=create-application-credential-detail#application-credentials

API methods to add:

• GET (show, list)
• POST (create)
• DELETE (delete)

Desired package path: gophercloud/openstack/identity/v3/extensions/applicationcredentials

struct draft:

type ApplicationCredential struct {
        ID           string `json:"id"`
        Name         string `json:"name"`
        Description  string `json:"description"`
        Unrestricted bool   `json:"unrestricted"`
        // Secret can be computed and returned by an API only once during the POST request
        Secret    string       `json:"secret"`
        ProjectID string       `json:"project_id"`
        // only role ID or/and role name could be sent in the POST request
        Roles     []roles.Role `json:"roles"`
        ExpiresAt string       `json:"expires_at"`
}

[jtopjian]

@kayrus I'm doing a trial of allowing people to do full CRUD in a single PR. Feel free to go ahead if you'd like.

Small nit, but ExpiresAt should be time.Time.

Also, with regard to Roles, if you're able to re-use the Roles struct from the roles package, go for it. But if it doesn't work for some reason, you can create a applicationcredentials.Roles struct.

[kayrus]

@jtopjian glad to hear that. Indeed, Role struct from the role package is not suitable, I had to introduce independent one.

[kayrus]

@jtopjian, looks like the application credential is not an extension, but a built-in auth method. It is descripbed directly inside the https://developer.openstack.org/api-ref/identity/v3/, despite of extension auth methods like OAuth1 or Trusts. Therefore I'd consider to put it directly into the gophercloud/openstack/identity/v3 path.