Allow .NET SDK versions to be provided with `--platform-version` argument

[cormacpayne]

Currently, only runtime versions can be provided to the --platform-version argument for oryx build, preventing users from specifying a specific version of the .NET SDK they'd like to build with (instead they'd need to know the corresponding runtime version shipped with the SDK version they want to use).

This PR cleans up the logic around the version that can be provided and detected to account for SDK versions, while still respecting the runtime version logic we currently have in place.

• [x] The purpose of this PR is explained in this message or in an issue. If an issue please include a reference as #<issue_number>.
• [x] Tests are included and/or updated for code changes. ~- [ ] Proper license headers are included in each file.~

[daniv-msft]

@Cormacpayne Overall looks good to me, but I would prefer not to push it before .NET 8 GA. Do you see any urgency in getting this one in?