iproto: stringify IPROTO_RAFT, IPROTO_PROMOTE

This allows `xlog` Lua module to decode appropriate
types into symbolic form.

For example with the patch we should see raft and
promote types in output.

 | $ tarantoolctl cat 00000000000000000004.xlog
 | ---
 | HEADER:
 |   lsn: 2
 |   group_id: 1
 |   type: RAFT
 |   timestamp: 1621541912.4588
 | BODY:
 |   0: 3
 |   1: 4
 | ---
 | HEADER:
 |   lsn: 1
 |   replica_id: 4
 |   type: PROMOTE
 |   timestamp: 1621541912.4592
 | BODY:
 |   2: 0
 |   3: 0
 |   83: 3

Fixes #6088

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>