Update processcreds.CredentialProcessResponse visibility to public (#4621)

In order to have a canonical type for encoding/decoding JSON for a credential
process, the visibility of this type needed to be public. This type has been
detailed in official AWS CLI documentation and is effectively part of the
tooling ecosystem's external interface.

Author: joshdk

aws/credentials/processcreds/provider.go

@@ -226,12 +226,24 @@ func NewCredentialsCommand(command *exec.Cmd, options ...func(*ProcessProvider))
 	return credentials.NewCredentials(p)
 }
 
-type credentialProcessResponse struct {
-	Version         int
-	AccessKeyID     string `json:"AccessKeyId"`
+// A CredentialProcessResponse is the AWS credentials format that must be
+// returned when executing an external credential_process.
+type CredentialProcessResponse struct {
+	// As of this writing, the Version key must be set to 1. This might
+	// increment over time as the structure evolves.
+	Version int
+
+	// The access key ID that identifies the temporary security credentials.
+	AccessKeyID string `json:"AccessKeyId"`
+
+	// The secret access key that can be used to sign requests.
 	SecretAccessKey string
-	SessionToken    string
-	Expiration      *time.Time
+
+	// The token that users must pass to the service API to use the temporary credentials.
+	SessionToken string
+
+	// The date on which the current credentials expire.
+	Expiration *time.Time
 }
 
 // Retrieve executes the 'credential_process' and returns the credentials.
@@ -242,7 +254,7 @@ func (p *ProcessProvider) Retrieve() (credentials.Value, error) {
 	}
 
 	// Serialize and validate response
-	resp := &credentialProcessResponse{}
+	resp := &CredentialProcessResponse{}
 	if err = json.Unmarshal(out, resp); err != nil {
 		return credentials.Value{ProviderName: ProviderName}, awserr.New(
 			ErrCodeProcessProviderParse,