feat(nvs): update to support string values

[finger563]

Description

• Add template specialization functions for reading/writing string values to NVS
• Update example to test new functions

Motivation and Context

Strings are useful basic elements to store in NVS. The underlying storage supports them, but the espp::Nvs class did not, since they require slightly different APIs. This PR updates the implementation to support string storage using those APIs.

How has this been tested?

Building and running the nvs/example on a QtPy ESP32s3.

Screenshots (if appropriate, e.g. schematic, board, console logs, lab pictures):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update
• [ ] Hardware (schematic, board, system design) change
• [x] Software change

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have added / updated the documentation related to this change via either README or WIKI

Software

• [x] I have added tests to cover my changes.
• [ ] I have updated the .github/workflows/build.yml file to add my new test to the automated cloud build github action.
• [x] All new and existing tests passed.
• [x] My code follows the code style of this project.

[github-actions[bot]]

:white_check_mark:Static analysis result - no issues found! :white_check_mark: