[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/channels/app/mention_parser_standard_test.go

@@ -105,7 +105,7 @@ func TestIsKeywordMultibyte(t *testing.T) {
 			post := &model.Post{
 				Message: tc.Message,
 				Props: model.StringInterface{
-					"attachments": tc.Attachments,
+					model.PostPropsAttachments: tc.Attachments,
 				},
 			}