feat(firebase_ui_auth): add autofocus property to PhoneInput widget

[orenagiv]

Description

When using the PhoneInput widget as described in the "Using widget" or "Building a custom widget" sections here, it usually means there are additional fields or other widgets on screen, and therefore the autofocus: true which is currently hard-coded into the PhoneInput widget might conflict with the desired UX of that screen.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
  Note: I see errors in firebase_ui_firestore (which also exist in the main branch), but no issues in the package which this PR relates to (firebase_ui_auth).
• All unit tests pass (melos run test:unit:all doesn't fail).
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[russellwheatley]

Hey @orenagiv - thank you for the PR, could you create a small example app demonstrating this use case, please?

[orenagiv]

Hey @russellwheatley
Sure - will send an example ASAP.

[orenagiv]

Hey @russellwheatley,

The example app can be found here:
https://github.com/orenagiv/firebase_ui_auth_example

You can use the following phone number and verification code to test the flow:

Test phone number: +1 650-555-1234
Verification code: 123456

The relevant Widget which demonstrates the usage of the Firebase UI Auth flow is:

widgets/sign_in_with_mobile.dart

Please let me know if you need anything else.

[russellwheatley]

@orenagiv - thank you! I'll take a look at some point this week👌