fix(core): Set userAgent when using AmplifyOutputs to configure

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:
The user agent is now configured when Amplify is using a Gen2 config.

How did you test these changes?

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(storage): message, feat(auth): message, chore(all): message)

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[mattcreaser]

https://github.com/aws-amplify/amplify-flutter/blob/1532f8419e0765ef290ecec484ea7c2ec4af5702/packages/amplify_datastore/android/src/main/kotlin/com/amazonaws/amplify/amplify_datastore/AmplifyDataStorePlugin.kt#L950 this will cause issues with this PR

This is because flutter first calls Amplify.addUserAgentPlatform which calls UserAgent.configure. Flutter then calls Amplify.configure which, after this PR, also calls UserAgent.configure. UserAgent.configure throws an exception if it's called multiple times.

As a workaround I removed the exception that's thrown if the user agent is configured multiple times, instead just silently failing on subsequent configures. This preserves the existing Flutter behaviour and allows the user agent to be set from Amplify.configure. To help customers avoid calling this method I'll annotate it with AmplifyInternalWarning.