Embedder: Refactor EmbedderConfigBuilder by backend

[cbracken]

Extracts backend-specific code in EmbedderConfigBuilder to separate translation units. In particular, this allows for less conditional header includes, and more specifically, for code relating to the Metal backend to include headers that include Objective-C types -- today we cast these all to void* to avoid declaring them in headers, which requires special handling for ARC.

An alternative approach would have been to extract backend-specific subclasses, but there are test suites such as EmbedderTestMultiBackend that are executed against multiple backends, which currently make that approach impractical, though that should likely be the long-term goal.

Issue: flutter/flutter#137801
Issue: flutter/flutter#158998

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[cbracken]

This isn't the final state I'd like to get this to, but is a first-pass to move backend-specific code out into separate translation units. Should make further refactoring a lot simpler and avoids a ton of ifdef-ing.