Add support for standalone sdktool installation

[luk1337]

This allows us to install sdktool using the following command:
aqt install-tool windows desktop sdktool

[miurahr]

thank you for the proposal!
when check combinations.json, there are only entries for sdktools.doc not sdktool
https://github.com/miurahr/aqtinstall/blob/master/aqt/combinations.json#L814

Could you also improve CI tool to update the data?
https://github.com/miurahr/aqtinstall/blob/master/ci/generate_combinations.py#L54

iter_tool_variants method seems return no "sdktool" variant.

A tool is called by GitHub Actions
https://github.com/miurahr/aqtinstall/blob/master/.github/workflows/check-combinations.yml

[luk1337]

It seems like ci/generate_combinations.py is already aware of sdktool without doing anything. However, it was failing at tools_qtdesignstudio_generation2, which I had to temporarily blacklist when running it.

[miurahr]

It seems like ci/generate_combinations.py is already aware of sdktool without doing anything. However, it was failing at tools_qtdesignstudio_generation2, which I had to temporarily blacklist when running it.

Thank you for the trial. You are welcome to change to add it blacklist in another PR.

[luk1337]

It seems like ci/generate_combinations.py is already aware of sdktool without doing anything. However, it was failing at tools_qtdesignstudio_generation2, which I had to temporarily blacklist when running it.

Thank you for the trial. You are welcome to change to add it blacklist in another PR.

Maybe tools_qtdesignstudio_generation2 will eventually get fixed, it just seems weird that file exists but .sha256 doesn't work.

• https://download.qt.io/online/qtsdkrepository/linux_x64/desktop/tools_qtdesignstudio_generation2/Updates.xml
• https://download.qt.io/online/qtsdkrepository/linux_x64/desktop/tools_qtdesignstudio_generation2/Updates.xml.sha256

[ddalcino]

Maybe tools_qtdesignstudio_generation2 will eventually get fixed, it just seems weird that file exists but .sha256 doesn't work.

* https://download.qt.io/online/qtsdkrepository/linux_x64/desktop/tools_qtdesignstudio_generation2/Updates.xml

* https://download.qt.io/online/qtsdkrepository/linux_x64/desktop/tools_qtdesignstudio_generation2/Updates.xml.sha256

Unfortunately, missing .sha256 files are common; it's an existing issue that does not have an easy solution (see #578). However, they are typically missing for less than 24 hours after new files are uploaded. If you check the sha256 link again, you'll find that it's working now.