api: provide api definition over api

This patch allows a client to bootstrap itself by downloading the
JSON API definitions over the API itself.

This patch enables it for Python (probably need a dynamic language).
Call VPPApiClient with the new bootstrapapi=True parameter.

Example (Python):

from vpp_papi import VPPApiClient
vpp = VPPApiClient(bootstrapapi=True)
rv = vpp.connect("foobar")
assert rv == 0
print(f'SHOW VERSION: {vpp.api.show_version()}')
vpp.disconnect()

Type: feature
Change-Id: Id903fdccc82b2e22aa1994331d2c150253f2ccae
Signed-off-by: Ole Troan <otroan@employees.org>

src/vlibapi/api_common.h

@@ -354,6 +354,8 @@ typedef struct api_main_t
 
   /** client message index hash table */
   uword *msg_index_by_name_and_crc;
+  /** plugin JSON representation vector table */
+  u8 **json_api_repr;
 
   /** api version list */
   api_version_t *api_version_list;