Add library.json for PlatformIO

[egnor]

PlatformIO is an increasingly popular build system for embedded software. (I am not affiliated with it in any way.) It has a library manager that can establish dependencies directly to github repositories,
given only a relatively simple library.json file with package metadata and build instructions. This is such a file.

[egnor]

Note, this sets -DMQTT_USE_CUSTOM_SOCKET_HANDLE by default, because any system using PlatformIO will not be covered by the default socket type. This can be overridden by the user if necessary.