[fuchsia] Add ability to configure separate data and asset dirs

[jamesr]

Description

This allows Fuchsia components executed by the Flutter runner to
specify a directory containing assets if they wish to store assets
separate from program data. This is specified in the program metadata
field within the component's specification with the new "assets"
attribute. If this attribute is absent, assets are loaded relative to
the path specified in the "data" attribute as before.

This is useful in the short term to use a location in the package where
we can store small files more efficiently. It is also potentially
useful longer term to enforce a stronger separatation between
executable program data and non-executable assets.

Related Issues

https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=53243

Tests

I added the following tests:

flutter_runner_tests now includes a component_unittest with testing for the program_metadata parsing aspect of this change.

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Breaking Change

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[jamesr]

I've added some basic tests, and manually performed integration tests with Fuchsia. I'm going through the internal Fuchsia patching guidelines to run further integration tests and will update this PR if those turn up any issues.

I spent some time trying to refactor the Application creation logic to be more testable and generally understandable. It's pretty resistant to both in its current state. I'd like to try to land that separately from functionality changes to minimize churn.

[jamesr]

I've queued up the internal tests in cl/316017532