mesh: Add MESH_PMKSA_GET/ADD commands

mesh: Add MESH_PMKSA_GET/ADD commands

These commnds are mesh version of PMKSA_GET/ADD commands. So the usage
and security risk is similar to them. Refer to
commit 3459381dd260e15e7bf768a75cb0b799cc1db33a ('External persistent
storage for PMKSA cache entries') also.

The MESH_PMKSA_GET command requires peer MAC address or "any" as an
argument and outputs appropriate stored PMKSA cache. And the
MESH_PMKSA_ADD command receives an output of MESH_PMKSA_GET and re-store
the PMKSA cache into wpa_supplicant. By using re-stored PMKSA cache,
wpa_supplicant can skip commit message creation which can use
significant CPU resources.

The output of the MESH_PMKSA_GET command uses the following format:
<BSSID> <PMKID> <PMK> <expiration in seconds>

The example of MESH_PMKSA_ADD command is this.
MESH_PMKSA_ADD 02:00:00:00:03:00 231dc1c9fa2eed0354ea49e8ff2cc2dc cb0f6c9cab358a8146488566ca155421ab4f3ea4a6de2120050c149b797018fe 42930
MESH_PMKSA_ADD 02:00:00:00:04:00 d7e595916611640d3e4e8eac02909c3c eb414a33c74831275f25c2357b3c12e3d8bd2f2aab6cf781d6ade706be71321a 43180

This functionality is disabled by default and can be enabled with
CONFIG_PMKSA_CACHE_EXTERNAL=y build configuration option.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>