Ble read/write characteristics over MQTT. (#988)

* EXPERIMENTAL: Ble read/write characteristics over MQTT.

Uses #979

This allows reading and writing BLE characteristics from an MQTT message.

Example format:
mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoBT/config -m '{"ble_write_address":"AA:BB:CC:DD:EE:FF", "ble_write_service":"cba20d00-224d-11e6-9fb8-0002a5d5c51b", "ble_write_char":"cba20002-224d-11e6-9fb8-0002a5d5c51b", "ble_write_value":"TEST", "ttl":4 }'
mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoBT/config -m '{"ble_read_address":"30:AE:A4:7C:3C:A6", "ble_read_service":"cba20d00-224d-11e6-9fb8-0002a5d5c51b", "ble_read_char":"cba20002-224d-11e6-9fb8-0002a5d5c51b", "ttl": 2 }'
The ttl parameter is the number of calls to connect (defaults to 1), which occur after the BLE scan completes.

A response is provided over MQTT in the format:
write : {"id":"30:AE:A4:7C:3C:A6","service":"cba20d00-224d-11e6-9fb8-0002a5d5c51b","characteristic":"cba20002-224d-11e6-9fb8-0002a5d5c51b","write":"TEST","success":true}
read : {"id":"30:AE:A4:7C:3C:A6","service":"cba20d00-224d-11e6-9fb8-0002a5d5c51b","characteristic":"cba20002-224d-11e6-9fb8-0002a5d5c51b","read":"TEST","success":true}

* Add generic BLE connect class and read data as hex.

* Add value_type parameter and document usage

* Add note to docs

main/config_BT.h

@@ -46,6 +46,7 @@ bool bleConnect = AttemptBLECOnnect;
 #  ifndef BLE_FILTER_CONNECTABLE
 #    define BLE_FILTER_CONNECTABLE 1
 #  endif
+#  include "NimBLEDevice.h"
 #endif
 
 /*----------------------BT topics & parameters-------------------------*/
@@ -151,6 +152,7 @@ enum ble_sensor_model {
   IBT4XS,
   DT24,
   EDDYSTONE_TLM,
+  GENERIC,
 };
 
 /*-------------------PIN DEFINITIONS----------------------*/
@@ -168,6 +170,26 @@ enum ble_sensor_model {
 #endif
 
 /*---------------INTERNAL USE: DO NOT MODIFY--------------*/
+#ifdef ESP32
+enum ble_val_type {
+  BLE_VAL_STRING = 0,
+  BLE_VAL_HEX,
+  BLE_VAL_INT,
+  BLE_VAL_FLOAT,
+};
+
+struct BLEAction {
+  std::string value;
+  char addr[18];
+  NimBLEUUID service;
+  NimBLEUUID characteristic;
+  bool write;
+  bool complete;
+  uint8_t ttl;
+  ble_val_type value_type;
+};
+#endif
+
 struct BLEdevice {
   char macAdr[18];
   bool isDisc;