API - CRUD for peers, interfaces and users (#340)

Public REST API implementation to handle peers, interfaces and users. It also includes some simple provisioning endpoints.

The Swagger API documentation is available under /api/v1/doc.html

internal/app/api/v1/models/models.go

@@ -0,0 +1,8 @@
+package models
+
+// Error represents an error response.
+type Error struct {
+	Code    int    `json:"Code"`              // HTTP status code.
+	Message string `json:"Message"`           // Error message.
+	Details string `json:"Details,omitempty"` // Additional error details.
+}