Fixes to help integrating CoAP-simple-library in other projects

[jackjansen]

This pull request contains one bug fix:

• payloadlen was uint8_t in packet declaration, so packets could never have a payload bigger than 255 bytes. Changed to size_t everywhere.

In addition there are a number of fixes to help using this module in other packages:

• Type callback changed to CoapCallback so it does not conflict with identifiers used in other packages.
• Various defined size constants prefixed with COAP_ so they don't conflict, and made overridable using compiler flags.
• Use a std::function in stead of a function pointer for callbacks. This makes integration with C++ methods a lot easier (and should be backward compatible).
• Added a library.json to enable easy use of this package in platformio

[hirotakaster]

Hi @jackjansen , Thank you for your PR. I will test.