Moved C++ headers to minio-cpp subdirectory

[kobalicek]

• Use #include <minio-cpp/header.h> to include minio-cpp now
• Header files have consistent guards that don't start with _
• Added a SPDX license identifier to each source and header file

[kobalicek]

@balamurugana Can you review again?

I have changed minio-cpp to miniocpp as required although I think it's ugly...