[WIP] Added autofill functionality

[ganttastic]

This PR adds autofill functionality to the engine.

iOS - functionally complete, but has a bug where autofill only shows for non-default keyboards.
Android - still needs work and guidance. Current work could be removed if only iOS implementation is wanted in this PR.
Web - not started

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

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What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
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[ganttastic]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[googlebot]

CLAs look good, thanks!

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[ganttastic]

Unsure if this is the correct view to set the auto fill hints on.

[isaiahtaylorhh]

Hello, thanks for your work on this! I'd like to cast my vote in favor of getting this accepted with the iOS capability only and opening a separate PR for the other channels. Unless there is a near timeline for the whole thing being complete. Thank you!

[ganttastic]

Full autofill implementation is in PR #17493.