[google-maps] allow to show loading marker when remote marker is not loaded yet

## Feature Request

### Plugin
capacitor/google-maps

### Description
In an app where all markers are loaded from a remote source, the current issue with remote markers is, that the app shows either "no icon" or the "default" red icon until the remote marker is loaded. When a lot of markers are loaded, it will look like a bug for the user who uses the Google Maps app. We want to avoid this kind of marker flash.

### Platform(s)
android, ios

### Preferred Solution
The preferred solution is that the developer can set a loading marker until the content is loaded or replace the default marker with a custom marker that is already built on the device.

### Alternatives
- Alternatively, the plugin can be fixed so as not to display the default red marker.