Enable clang-tidy to scan druid header files.

**Is your feature request related to a problem? Please describe.**
Compiler support for modules isn't 100% in the instances we need to use headers we will need clang-tidy to scan these for errors.

**Describe the solution you'd like**

- [ ] Update .clang-tidy file to filter for .h along with .ixx files.
- [ ] Fix errors in any header files (may require NOLINT).

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.