feat: library definition for PlatformIO

[obones]

Many embedded projects are using PlatfomIO as a development environment, in particular in the ESP32 world.

It is possible to use third party libraries via such a definition in the project platform.io file:

lib_deps = 
    https://github.com/boblemaire/asyncHTTPrequest

For this to work, the referenced project must include a library.json file describing the library and its folder layout.

This PR provides the file so that flatbuffers is usable as a dependency in PlatformIO, as requested by #7189

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