* Migrate a few last places to GrDirectContext

This is a followup to #19962 to cover a few places where we
were still using GrContext. No functional impact.

* Formatting

Refactors the Windows embedding internals to make an engine object that
owns things associated with the engine rather than the view, and updates
the API surface to allow using the engine directly.

This is an incremental step toward both a cleaner, non-struct-based
internal structure and a finalized API surface.

* add supported luci builders

* fix license

* add readme

* add code to validate json contents

* license

* move dev to ci

* fix license

* add shell script to run test

These are now flagged by the Dart analyzer.

This reverts commit 4126c066.

The LUCI recipe issue should have already been fixed by @christopherfujino 

Try to trigger the LUCI tests with this as the "retry" button is somehow disabled for https://ci.chromium.org/p/flutter/builders/prod/Linux%20Fuchsia/4755 even if I logged in.

This reverts commit 00e75747.

Try to fix the LUCI Fuchsia build.

- Rolls buildroot to pick up
  https://github.com/flutter/buildroot/pull/389
- Adds support to the gn wrapper for setting the traget CPU on Windows

Attempting to generate Windows arm64 GN files will currently fail, due
to lack of arm64 support for an OpenGL test support target, but this
sets the foundation, and makes experimentation easier.

Part of https://github.com/flutter/flutter/issues/62597

Instead of a hand-rolled discriminated union (originally used to avoid a C++17
dependency, which is no longer an issue), implement EncodableValue as a
std::variant. Rather than simply changing the internals, this makes EncodableValue
a minimal std::variant subclass with only a handful of added methodS, replacing
the old IsFoo/FooValue APIs with the standard std::holds_alternative/std::get,
so that plugin code will use a standard-based API rather than a Flutter-specific
API for wrapped values.

This is a breaking change for Windows and GLFW plugins. In the short
term USE_LEGACY_ENCODABLE_VALUE can be set in builds to use the old
version, to separate rolling from updating.

Fixes https://github.com/flutter/flutter/issues/61970

The fuchsia code around metrics and sizing was just sending this
information through a side-channel, when the engine already had the
information available. So, delete all of it to make future CLs simpler.

Additionally, the SceneUpdateContext has many unneccesary dependencies
re: metrics and PaintTasks. Break those to make future CLs simpler.

Tested: Ran all unittests and ran workstation on Fuchsia
BUG: 53062, 53063