Feature: support "dry run" mode

[pzhlkj6612]

Is your feature request related to a problem? Please describe.

I'm going to see what aqt will do after getting metadata, but I don't want it to download any tarballs.

Describe the solution you'd like

$ aqt install-qt linux desktop 6.* --dry-run
INFO    : aqtinstall(aqt) v...
INFO    : Resolved spec '6.*' to 6.6.1
INFO    : Downloading qtbase... Skipped as --dry-run is set (size ..., checksum ...)
INFO    : Downloading qtdeclarative... Skipped as --dry-run is set (size ..., checksum ...)
INFO    : Downloading qtsvg... Skipped as --dry-run is set (size ..., checksum ...)
INFO    : Downloading qttools... Skipped as --dry-run is set (size ..., checksum ...)

Describe alternatives you've considered

None

Additional context

None