Forward declare 'struct ast_json' in asterisk.h

The ast_json structure is used in many Asterisk headers and is often the
only part of json.h used.  This adds a forward declaration to asterisk.h
and removes the include of json.h from many headers.  The declaration
has been left in endpoints.h and stasis.h to avoid problems with source
files that use ast_json functions without directly including json.h.

ari.h continues to include json.h as it uses enum
ast_json_encoding_format.

Change-Id: Id766aabce6bed56626d27e8d29f559b5e687b769