Fix Swift build warnings

[matt-livefront]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [X] Other

Objective

This fixes a few build warnings when integrating the Swift SDK package within an app:

ld: warning: dylib (libbitwarden_uniffi.dylib) was built for newer iOS Simulator version (16.2) than being linked (15.0)

It looks like Rust probably uses the latest iOS deployment target/min iOS version if not specified. I added IPHONEOS_DEPLOYMENT_TARGET="13.0" to set this to iOS 13, which matches the version in Package.swift.
https://github.com/bitwarden/sdk/blob/c707fc9ff9a241b73ef7471f90a3d0b0e68c6e33/languages/swift/Package.swift#L9

ld: warning: linking against a dylib which is not safe for use in application extensions: libbitwarden_uniffi.dylib

We need to link the SDK within an iOS app extension to use for auto fill. RUSTFLAGS="-C link-arg=-Wl,-application_extension" enables a linker flag which specifies that this code is being linked for use in an extension. This prevents using some UIKit APIs that don't exist in extensions but shouldn't have any negative effects on the SDK.

There's a bunch of build warnings from the SDK. These all appear to be around how Rust escapes argument parameters in the generated code, so it seems safe enough to ignore these and add the -suppress-warnings Swift flag. Xcode does a pretty good job of hiding warnings from SPM packages, but they still show up in build logs so it would be nice to hide these.

Code changes

• Package.swift: Adds a flag to suppress warnings from the SDK package.
• build.sh: Adds two environment variables to fix build warnings.

[Hinton]

Thanks @matt-livefront.

This all looks good, I've triggered the GH actions and will merge once they complete.