WIP: TextInputTheme support step 1 of 3.

[darrenaustin]

Description

As part of the larger theme system update, this PR provides a new TextFieldTheme that configures the properties of TextFields. The initial supported properties are the cursorColor and selectionHandleColor, with more to come in future PRs.

As we are updating the default values for each of these properties this will eventually be a breaking change. However, for this first step this is all turned off by default. To turn on the text field theme support you can set the new temporary ThemeData.useTextFieldTheme flag to true.

TBD: description of new defaults and breaking change migration docs

Related Issues

#17635
#56082

Tests

I added tests for the new theme as well as updating the overall ThemeData tests.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them?

This PR specifically won't break any tests, as it is a soft migration. Overall however, after all stages of this are complete it will be a breaking change.

• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template Replace this with a link to a pull request that adds the migration guide to https://flutter.dev/docs/release/breaking-changes

TBD: breaking change docs.