FR: Support Google Tag Manager Server-side by allowing changes to GTAG_URL

[CTassisF]

[REQUIRED] Describe your environment

• Operating System version: N/A
• Browser version: N/A
• Firebase SDK version: 8.6.8
• Firebase Product: analytics

[REQUIRED] Describe the problem

Add support to Google Tag Manager Server-side by allowing changes to GTAG_URL before initializing Firebase Analytics.

More information about Tag Manager Server-side can be found here: https://developers.google.com/tag-manager/serverside