Support ADXL345 sensor (digital accelerometer)

**Describe the problem you have/What new integration you would like**

Support i2c ADXL345 sensor (digital accelerometer)

**Please describe your use case for this integration and alternatives you've tried:**

Got it working using custom sensor
```
#include "esphome.h"
#include
#include
#include

class ADXL345Sensor : public PollingComponent {
public:
Adafruit_ADXL345_Unified accel = Adafruit_ADXL345_Unified(12345);
Sensor *accel_x_sensor = new Sensor();
Sensor *accel_y_sensor = new Sensor();
Sensor *accel_z_sensor = new Sensor();

ADXL345Sensor() : PollingComponent(1000) { }

void setup() override {
accel.begin();
/* Set the range to whatever is appropriate for your project */
// accel.setRange(ADXL345_RANGE_16_G);
accel.setRange(ADXL345_RANGE_8_G);
// accel.setRange(ADXL345_RANGE_4_G);
// accel.setRange(ADXL345_RANGE_2_G);
//
}

void update() override {
sensors_event_t event;
accel.getEvent(&event);

accel_x_sensor->publish_state(event.acceleration.x);
accel_y_sensor->publish_state(event.acceleration.y);
accel_z_sensor->publish_state(event.acceleration.z);
}

};

```

```
esphome:
[...]
includes:
- ADXL345.h
libraries:
- "Grove 3-Axis Digital Gyro"
- platform: custom
lambda: |-
auto adxl345 = new ADXL345Sensor();
App.register_component(adxl345);
return { adxl345->accel_x_sensor, adxl345->accel_y_sensor, adxl345->accel_z_sensor };
sensors:
- name: "Acceleration X"
- name: "Acceleration Y"
- name: "Acceleration Z"
```

**Additional context**

[Adafruit-ADXL345](https://platformio.org/lib/show/15/Adafruit-ADXL345-Unified)