add initial support for sending a capability hash

[ailin-nemui]

[fschlich]

src/core/xep/xep.c needs to #include caps.h, and perhaps a call to caps_deinit() as well, no?

xep/xep.c: In function ‘xep_init’:
xep/xep.c:36:9: warning: implicit declaration of function ‘caps_init’; did you mean ‘xep_init’? [-Wimplicit-function-declaration]
   36 |         caps_init();
      |         ^~~~~~~~~
      |         xep_init