refactor: switch to OpenAPITools's generator for our HTTP clients (#33)

* build: add CI job to check OpenAPI generation is clean
* feat: add hidden flag for HTTP debug

internal/api/error.go

@@ -0,0 +1,22 @@
+package api
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Extension to let our API error type be used as a "standard" error.
+func (o *Error) Error() string {
+	if o.Message != "" && o.Err != nil {
+		var b strings.Builder
+		b.WriteString(o.Message)
+		b.WriteString(": ")
+		b.WriteString(*o.Err)
+		return b.String()
+	} else if o.Message != "" {
+		return o.Message
+	} else if o.Err != nil {
+		return *o.Err
+	}
+	return fmt.Sprintf("<%s>", o.Code)
+}