REORG: include: create new file haproxy/api-t.h

REORG: include: create new file haproxy/api-t.h

This file is at the lowest level of the include tree. Its purpose is to
make sure that common types are known pretty much everywhere, particularly
in structure declarations. It will essentially cover integer types such as
uintXX_t via inttypes.h, "size_t" and "ptrdiff_t" via stddef.h, and various
type modifiers such as __maybe_unused or ALIGN() via compiler.h, compat.h
and defaults.h.

It could be enhanced later if required, for example if some macros used
to compute array sizes are needed.