Add Flutter build rules

[cbracken]

Users should be able to build Flutter apps using Bazel. The Flutter tool doesn't yet have a stable API but if we get to that point, it would be reasonable to want to support that here, though we'd want to limit this to stable channel releases to have any hope of keeping up with flutter_tools changes.

Target platform	Host platforms
iOS	macOS
Android	Linux, macOS, Windows*
Linux	Linux
macOS	macOS
Windows	Windows*

*Building on Windows hosts is not yet supported, but if support is ever added to these rules, we could add support for Flutter as well.

There are a couple options for how to proceed -- do this entirely using flutter_tools, or do the Flutter-specific building using the tool, and take a dependency on existing iOS and Android build rules for the iOS and Android apps.

[cbracken]

Related context:

• https://groups.google.com/d/msg/bazel-discuss/SnXvMgAUGzI/WAN7R0S2BwAJ
• Dart rules bazelbuild/bazel#10207
• Some Flutter users have expressed they want Bazel support flutter/flutter#19680
• flutter/engine does not build with Bazel flutter/flutter#14125

[zachgrayio]

Hey @cbracken - thanks for the ping. We'll TAL!