[MM-62427] Add message attachments validation (#30180)

* Add message attachments validation

* Add props validation

* Validate slack attachment fields

* Update tests and library usage

* Improve interactive dialog error for length checks

* Allow predefined colors for slack attachments

* Fix TestPostAction

* Use const for data source

* Add tests

* Cleanup unused props

* Add happy path tests

* lint fixes

* Add validation for PostActionOptions

server/public/pluginapi/experimental/bot/poster/default_poster_test.go

@@ -106,7 +106,7 @@ func TestDMWithAttachments(t *testing.T) {
 		ChannelId: dmChannelID,
 		Type:      model.PostTypeSlackAttachment,
 		Props: model.StringInterface{
-			"attachments": attachments,
+			model.PostPropsAttachments: attachments,
 		},
 	}