Add instance metadata to gfx-replay.

[0mdc]

Motivation and Context

This changeset adds an InstanceMetadata entry to gfx-replay keyframes so that this information can be made available to external players.

This information can be used to do commands on gfx-replay instances based on real simulation IDs, for example:

• Hiding an agent.
• Highlighting an object's mesh.
• Tooling.
• ...

How Has This Been Tested

• Added test.
• Tested in Unity gfx-replay client.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[jturner65]

This looks good, thanks.