This adds Obj-C/Swift bridging headers to Swift targets, which allows us
to use public Flutter framework API from Swift code in the framework.

This also restricts visibility of the Swift framework targets to the
corresponding framework target. The addition of a bridging header allows
us to use public framework types declared in those headers from Swift,
but those types are defined in the framework target, and thus using the
Swift target directly may result in linking errors in the user. Instead,
all usage of the Swift target should be transitive via the framework
target.

Issue: #144791

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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