Explicit convert from std::filesystem::path to std::string for Windows compatibility

[traversaro]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	No ticket.
Primary OS tested on	Windows
Robotic platform tested on	Not tested on any robotic platform.
Does this PR contain AI generated software?	No

Description of contribution in a few bullet points

On Linux and macOS std::filesystem::path is implicitly convertible to std::string, while on Windows it is convertible to std::wstring, see https://en.cppreference.com/w/cpp/filesystem/path and https://stackoverflow.com/questions/57377349/implicit-conversion-between-stdfilesystempath-and-stdstring-should-it-hap . If we want to convert std::filesystem::path to std::string in a Windows-compatible way, it is then necessary to explicitly call the .string() method.

Description of documentation updates required from your changes

Description of how this change was tested

I checked if the program compiled.

Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
...v2_waypoint_follower/plugins/photo_at_waypoint.cpp	91.52% <100.00%> (ø)

... and 4 files with indirect coverage changes