Add warning for --framework option on solution builds

Manual testing confirms the feature works correctly:
- Using --framework on solution shows NETSDK1234 warning
- Using --property:TargetFramework does not show warning
- Warning appears for build, clean, publish, and test commands

Changes:
- Added NETSDK1234 warning string in Strings.resx
- Modified FrameworkOption to set _CommandLineDefinedTargetFramework property
- Added _CheckForSolutionLevelTargetFramework target in Microsoft.NET.Sdk.Solution.targets

Co-authored-by: marcpopMSFT <12663534+marcpopMSFT@users.noreply.github.com>

Author: Copilot