mesh: add channel switch function

mesh: add channel switch function

1. add network channel switch function.
   - users can obtain the new channel through the event MESH_EVENT_CHANNEL_SWITCH.
   - the entire network will be moved to the same channel as the router without user intervention.
   - if no router is in the network, users can call esp_mesh_switch_channel() on the root side to
     move the entire network　to their desired channel.
2. support not configuring the network channel.
3. support not configuring the router BSSID even if the router is hidden.
4. add allow_channel_switch to mesh configuration.
   - if the channel is not specified, this value will be ignored.
   - if the channel is specified and this value is set, when "fail" (mesh_attempts_t) times of parent selection
     or look for networks are reached, device will change to perform a full channel scan for networks that could join.
   - if the channel is specified and this value is set to 0, when a root is not elected, channel switch is not allowed.
     but when a root appears, the root performs a full channel scan during the process of connecting to the router,
     so even allow_channel_switch is set to 0, the root may still switch channel and eventually the entire network
    changes channel.
5. add allow_router_switch to mesh router configuration.
   - if the BSSID of router is not specified, this value will be ignored.
   - if the BSSID of router is specified and this value is set, when the router of this specified BSSID fails to be found
     after　"fail" (mesh_attempts_t) times, the entire network is allowed to switch to another router with the same SSID.
6. modify the root to perform a full channel scan when esp_wifi_connect().
7. support handling beacon without DS Paramter Set.