Make `GenericSingleTagWriter` public, like all the other generic writers/parsers/validators

[jaybpierce]

Change Notes

• Make GenericSingleTagWriter public, like all the other generic writers/parsers/validators
• Needed to allow adding custom HLSTagDescriptors

Pre-submission Checklist

• [x] I ran the unit tests locally before checking in.
• [x] I made sure there were no compiler warnings before checking in.
• [ ] I have written useful documentation for all public code.
• [ ] I have written unit tests for this new feature.