Adds `fetch-deps` support for:
- iOS/macOS dependencies, using `pod install`
- Dart package dependencies, using `pub get`
To make avoid doing extra work in the Dart dependencies step when using this with `*_platform_tests` CI, also adds flags for all of the other platforms, and adds a flag that allows skipping Dart dependencies for any package that doesn't have an example supporting any requested platform. This means that we can pass, e.g., `--windows --supporting-target-platforms-only` to only fetch Dart packages for packages with examples that will be build during the build-and-drive Windows tests.
Adds this as a new step in every platform tests CI task, and in the standard analyze step, so that we will pre-fetch Dart packages (and for iOS/macOS, pods). This won't yet fully eliminate later network access (see https://github.com/flutter/flutter/issues/131204), but will give us early warning on any major failures, such as pub being entirely unreachable from the bots.
- These are marked as an infrastructure step; we'll have to see if this ends up being confusing in practice. If `pub` resolution fails for legitimate reasons, such as a PR that tries to require a version of a package that doesn't exist or that has conflicts, this will cause a failure that is marked as infra. My assumption is that the much more common case is going to be that it is actually an infra failure.
Fixes https://github.com/flutter/flutter/issues/130280
