feat(file_system): update so that mount point and related const APIs are static

[finger563]

Description

Move various const APIs in espp::FileSystem to be static instead, so they can be used without initializing the filesystem.

Motivation and Context

There are use cases where you may want to get the mount point or access some of the APIs of the file system which do not require it to have been initialized without initializing the file system. This PR updates some const APIs to be static allowing such use.

How has this been tested?

Building and running the file_system/example on 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

• [ ] 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: