Add optional "precache" flag

[bartekpacia]

resolves #256 (what a nice round number!)

[bartekpacia]

Ping @subosito. I think this implementation is good enough (it surely is for my use case :) ). flutter precache is smart enough to only download artifacts for platforms which can be targeted from the current one - i.e. it won't download ios and macos artifacts on a Linux machine. More details: flutter/flutter#144656

[bartekpacia]

I just realized that this PR doesn't make sense. The archives that this action downloads already contain all the relevant artifacts:

For example take a look at contents of these directories:

• flutter_linux_3.19.0-stable.tar.xz comes with
• flutter_macos_arm64_3.19.0-stable.zip

These Linux archive already contain:

• bin/cache/artifacts/material_fonts
• bin/cache/artifacts/engine/android-arm64 (with -profile and -release variants)
  and so on

The macOS archive also contains macOS- and iOS-specific stuff like bin/cache/artifacts/ios-deploy/ios-deploy.