improve ContextManager

[bollhals]

This PR improves the performance of the ContextManager and enables nullable reference types for this file. (Info: Nullable reference types only make a difference where it is enabled, so this is safe to do)

PR with parts of https://github.com/SpecFlowOSS/SpecFlow/pull/2373

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] Performance improvement
• [x] Refactoring (so no functional change)
• [ ] Other (docs, build config, etc)

Checklist:

• [ ] I've added tests for my code. (most of the time mandatory)
• [x] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.