Add CI to detect public API changes in Swift

[paulb777]

• Update Danger to turn on GitHub public tag
• Generate a report in PR for public API changes

Both of these are likely using DocC to get the API docs

[ryanwilson]

Update: DocC likely isn't necessary for this.

# Generate symbol graphs from Swift Package manifest. From the root of the repo:
mkdir -p .build/symbol-graphs
swift build --target FirebaseMLModelDownloader -Xswiftc -emit-symbol-graph -Xswiftc -emit-symbol-graph-dir -Xswiftc .build/symbol-graphs

A JSON file will exist at .build/symbol-graphs/FirebaseMLModelDownloader.symbols.json that contains the API surface.

[sunmou99]

@yifanyang Please take a look at this issue

[paulb777]

Fixed by @sunmou99