[MM-25647]: ability to permanently delete users (#14944)

Summary:
    Config option to allow permanent user deletion ServiceSettings.EnableAPIUserDeletion
    Expose permanent user deletion through API
    Local mode for delete user for use in mmctl

Ticket Link:
    Server part of https://mattermost.atlassian.net/browse/MM-25647

model/client4.go

@@ -1240,6 +1240,16 @@ func (c *Client4) DeleteUser(userId string) (bool, *Response) {
 	return CheckStatusOK(r), BuildResponse(r)
 }
 
+// PermanentDeleteUser deletes a user in the system based on the provided user id string.
+func (c *Client4) PermanentDeleteUser(userId string) (bool, *Response) {
+	r, err := c.DoApiDelete(c.GetUserRoute(userId) + "?permanent=" + c.boolString(true))
+	if err != nil {
+		return false, BuildErrorResponse(r, err)
+	}
+	defer closeBody(r)
+	return CheckStatusOK(r), BuildResponse(r)
+}
+
 // ConvertUserToBot converts a user to a bot user.
 func (c *Client4) ConvertUserToBot(userId string) (*Bot, *Response) {
 	r, err := c.DoApiPost(c.GetUserRoute(userId)+"/convert_to_bot", "")

model/config.go

@@ -327,6 +327,7 @@ type ServiceSettings struct {
 	DEPRECATED_DO_NOT_USE_ImageProxyURL               *string `json:"ImageProxyURL" mapstructure:"ImageProxyURL"`         // This field is deprecated and must not be used.
 	DEPRECATED_DO_NOT_USE_ImageProxyOptions           *string `json:"ImageProxyOptions" mapstructure:"ImageProxyOptions"` // This field is deprecated and must not be used.
 	EnableAPITeamDeletion                             *bool
+	EnableAPIUserDeletion                             *bool
 	ExperimentalEnableHardenedMode                    *bool
 	DisableLegacyMFA                                  *bool `restricted:"true"`
 	ExperimentalStrictCSRFEnforcement                 *bool `restricted:"true"`
@@ -699,6 +700,10 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 		s.EnableAPITeamDeletion = NewBool(false)
 	}
 
+	if s.EnableAPIUserDeletion == nil {
+		s.EnableAPIUserDeletion = NewBool(false)
+	}
+
 	if s.ExperimentalEnableHardenedMode == nil {
 		s.ExperimentalEnableHardenedMode = NewBool(false)
 	}