share font collections between spawn engines

[gaaclarke]

Description

Implemented sharing font collections between spawned engines.

Related Issues

fixes flutter/flutter#72024

Tests

I added the following tests:

EngineTest unit tests.

Checklist

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• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
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[gaaclarke]

Landing on red, the red is an infra failure timeout.