Super user invites (#483)

This PR brings user invites logic to the Management service
via HTTP API. 
The POST /users/ API endpoint creates a new user in the Idp
and then in the local storage. 
Once the invited user signs ups, the account invitation is redeemed.
There are a few limitations.
This works only with an enabled IdP manager.
Users that already have a registered account can't be invited.

management/server/idp/util.go

@@ -1,6 +1,18 @@
 package idp
 
-import "encoding/json"
+import (
+	"encoding/json"
+	"math/rand"
+	"strings"
+)
+
+var (
+	lowerCharSet   = "abcdedfghijklmnopqrst"
+	upperCharSet   = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	specialCharSet = "!@#$%&*"
+	numberSet      = "0123456789"
+	allCharSet     = lowerCharSet + upperCharSet + specialCharSet + numberSet
+)
 
 type JsonParser struct{}
 
@@ -11,3 +23,37 @@ func (JsonParser) Marshal(v interface{}) ([]byte, error) {
 func (JsonParser) Unmarshal(data []byte, v interface{}) error {
 	return json.Unmarshal(data, v)
 }
+
+// GeneratePassword generates user password
+func GeneratePassword(passwordLength, minSpecialChar, minNum, minUpperCase int) string {
+	var password strings.Builder
+
+	//Set special character
+	for i := 0; i < minSpecialChar; i++ {
+		random := rand.Intn(len(specialCharSet))
+		password.WriteString(string(specialCharSet[random]))
+	}
+
+	//Set numeric
+	for i := 0; i < minNum; i++ {
+		random := rand.Intn(len(numberSet))
+		password.WriteString(string(numberSet[random]))
+	}
+
+	//Set uppercase
+	for i := 0; i < minUpperCase; i++ {
+		random := rand.Intn(len(upperCharSet))
+		password.WriteString(string(upperCharSet[random]))
+	}
+
+	remainingLength := passwordLength - minSpecialChar - minNum - minUpperCase
+	for i := 0; i < remainingLength; i++ {
+		random := rand.Intn(len(allCharSet))
+		password.WriteString(string(allCharSet[random]))
+	}
+	inRune := []rune(password.String())
+	rand.Shuffle(len(inRune), func(i, j int) {
+		inRune[i], inRune[j] = inRune[j], inRune[i]
+	})
+	return string(inRune)
+}