Add ability to install runtime only

[nikolai-laevskii]

Description:
Added ability to select to install runtime only instead of SDK. It utilizes built-in features of the dotnet-install script by conditionally passing -Runtime/--runtime flag.

Related issue:
actions#352

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[dsame]

I approved the whole PR but still have a question: what was a reason to move DotnetVersionResolver out of DotnetCoreInstaller? It doesn't seem to use any benefits of being instantiated in the run but brings a bunch of not-necessary changes in the tests.

[nikolai-laevskii]

In this particular PR it was done primarily to keep it in sync with the other PR: #19 so there wouldn't be so many conflicts to resolve and they wouldn't be too complicated. In general, syncing them like that, might not be a good idea, but because setup-dotnet doesn't get that many updates, it's just makes things a bit easier without any significant drawbacks.

As per change itself, it removes hard-coupled dependency on DotnetVersionResolver and makes implementation more flexible when it comes to introducing changes.

1. DotnetVersionResolver can now have additional parameters and they won't have to be passed through DotnetCoreInstaller
2. DotnetVersionResolver logic and API can now be altered without introducing changes to DotnetCoreInstaller
3. DotnetCoreInstaller doesn't need to have version resolution logic inside it as it only works with specific version format. Removing such logic makes DotnetCoreInstaller simpler and easier to maintain.

[VAllens]

This switch is cool, but I have another opposite idea, add a sdk-only switch.
Right now by default the runtime is always installed first, then the sdk.
When one wishes to install sdk only, there's really no need to install runtime separately before, since sdk already includes runtime.