[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/plugin_api_test.go

@@ -1520,7 +1520,7 @@ func TestPluginCreatePostAddsFromPluginProp(t *testing.T) {
 
 	actualPost, err := api.GetPost(post.Id)
 	require.Nil(t, err)
-	assert.Equal(t, "true", actualPost.GetProp("from_plugin"))
+	assert.Equal(t, "true", actualPost.GetProp(model.PostPropsFromPlugin))
 }
 
 func TestPluginAPIGetConfig(t *testing.T) {