Add ability to install just the runtime, rather than the SDK

[thomaslevesque]

Description:
This action sets up the .NET SDK, but in some cases I'd like to install just a specific runtime version, not the whole SDK.

Justification:
I have a library with tests for multiple .NET versions. I want to use the latest SDK, but to be able to run the tests for all the supported .NET versions, I also need the appropriate runtime versions to be installed.

Are you willing to submit a PR?
Open to it, but would need some guidance, since I'm not familiar with the code.

Note: this seems to be supported by the install.sh and install.ps1 scripts, but I see no way to pass the appropriate parameters to the script when using the action. Am I missing something?

[IvanZosimov]

Hi, @thomaslevesque 👋 Thank you for this feature request! We will investigate the possibility of adding this feature and get back to you with updates.

[ch1seL]

absolutely support, the most common case when building multitarget projects