Make GMS core optional

[TheOneWithTheBraid]

Describe the bug
Currently, user_location_plugin depends on the Google Play Services. In particular, gms-core:

classpath 'com.google.gms:google-services:4.2.0'

To Reproduce
Steps to reproduce the behaviour:

1. Add the plugin
2. Start your map view
3. See a friendly notice

Expected behaviour
To build applications running on all kinds of devices, especially in Asian regions and on privacy-enhanced devices, GMS should be at least an optional dependency.

Screenshots
whatever won't run without Google Play services, which are not supported by your device.

Smartphone (please complete the following information):

• Device: Google Pixel 4, flame
• OS: Android 11, Graphene OS

Additional context
Nope

[igaurab]

Sure i'll make it a priority and see what we can do about it. Thanks for the feedback