Add initial Microsoft Teams support

Documentation on https://github.com/42wim/matterbridge/wiki/MS-Teams-setup

vendor/github.com/yaegashi/msgraph.go/beta/ExternalItemModel.go

@@ -0,0 +1,15 @@
+// Code generated by msgraph-generate.go DO NOT EDIT.
+
+package msgraph
+
+// ExternalItem undocumented
+type ExternalItem struct {
+	// Entity is the base model of ExternalItem
+	Entity
+	// Properties undocumented
+	Properties *Properties `json:"properties,omitempty"`
+	// Content undocumented
+	Content *string `json:"content,omitempty"`
+	// ACL undocumented
+	ACL []ACL `json:"acl,omitempty"`
+}