FR: Analytics - Add support for logging app_info automatically

### [REQUIRED] Describe your environment

* Operating System version: N/A (macOS 10.14.6 if you're really interested)
* Browser version: Any (Safari 13.x, Chrome 80.x, Firefox, etc.)
* Firebase SDK version: 7.14.2
* Firebase Product: analytics

### [REQUIRED] Describe the problem
It's very critical for many business processes to be able to determine what app version certain events occurred with.

Currently, when events are logged from a Web platform to a new Analytics App+Web property, the app_info is entirely empty:

For other platforms (iOS/Android), that information is populated automatically with each event:

A suggestion would be to attempt to use the app version in a package.json file automatically (if it exists). Or, more universally useful, provide an API to allow the user of the SDK to set the app version at the beginning of the page's lifecycle (e.g., when first initializing firebase)