Implement onShowFileChooser, calling delegate

What changes are you making?

Implement onShowFileChooser(), calling delegate, for merchants with 3p apps installed which require that functionality.

How to test

Run the sample app against a store that uses id.me, try and submit a photo / file.

Before you merge

[!IMPORTANT]

[x] I've added tests to support my implementation

[x] I have read and agree with the Contribution Guidelines.

[x] I have read and agree with the Code of Conduct.

[x] I've updated the README.

Checklist for releasing a new version

- [x] I have bumped the version number in the [`build.gradle` file](https://github.com/Shopify/checkout-kit-android/blob/main/lib/build.gradle#L17) - [x] I have added a [Changelog](https://github.com/shopify/checkout-sheet-kit-android/blob/main/CHANGELOG.md) entry. - [x] I have updated the versions in the [README.md](https://github.com/shopify/checkout-sheet-kit-android/blob/main/README.md) for both Gradle and Maven.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.

Shopify / checkout-sheet-kit-android