feat(cli): Support creating/updating a Username-Password-Domain static credential (#1525)

Co-authored-by: Andrew Gaffney <andrew@gaffney.cc>
Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

api/credentials/custom.go

@@ -0,0 +1,44 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package credentials
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ParseUsernameDomain parses the username and domain from the given strings.
+// It returns the username, domain, and an error if any.
+// The username can be in the format of "username", "username@domain", or "domain\username".
+func ParseUsernameDomain(username, domain string) (string, string, error) {
+	switch {
+	case username != "" && domain == "":
+		atCount := strings.Count(username, "@")
+		if atCount > 1 {
+			return "", "", fmt.Errorf("invalid format, more than one '@' found")
+		}
+		backslashCount := strings.Count(username, "\\")
+		if backslashCount > 1 {
+			return "", "", fmt.Errorf("invalid format, more than one '\\' found")
+		}
+		if atCount > 0 && backslashCount > 0 {
+			return "", "", fmt.Errorf("invalid format, both '@' and '\\' found")
+		}
+
+		// username@domain
+		if u, d, ok := strings.Cut(username, "@"); ok {
+			return u, d, nil
+		}
+		// domain\username
+		if d, u, ok := strings.Cut(username, "\\"); ok {
+			return u, d, nil
+		}
+	case username != "" && domain != "":
+		if strings.Contains(username, "@") || strings.Contains(username, "\\") {
+			return "", "", fmt.Errorf("username and domain cannot be provided together with a username in the format of username@domain or domain\\username")
+		}
+	}
+
+	return username, domain, nil
+}