MM-13826 Webhooks: Allow "true"/"false" for bool values in payload body (#10114)

* Webhooks: Allow "true"/"false" for bool values in payload body

Some slack integrations encode bool fields as "true"/"false", which
was previously unsupported in mattermost due to how encoding/json works.
This commit adds an aliased type for bool that implements json.Unmarshaler
to maintain compatibility with Slack.

* Add missing copyright header to added files

model/slack_compatibility.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2019-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"fmt"
+	"strings"
+)
+
+// SlackCompatibleBool is an alias for bool that implements json.Unmarshaler
+type SlackCompatibleBool bool
+
+// UnmarshalJSON implements json.Unmarshaler
+//
+// Slack allows bool values to be represented as strings ("true"/"false") or
+// literals (true/false). To maintain compatibility, we define an Unmarshaler
+// that supports both.
+func (b *SlackCompatibleBool) UnmarshalJSON(data []byte) error {
+	value := strings.ToLower(string(data))
+	if value == "true" || value == `"true"` {
+		*b = true
+	} else if value == "false" || value == `"false"` {
+		*b = false
+	} else {
+		return fmt.Errorf("unmarshal: unable to convert %s to bool", data)
+	}
+
+	return nil
+}