Enable writing to a BLE characteristic

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

BLE Sensor only allow reading a characteristic, but I would like to also write to a characteristic in my application.

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

Since this is not possible today, I am not using BLE in my application.

**Additional context**

Start by locating the BLE Sensor implementation and its existing characteristic-reading path. Determine how a write operation should be exposed for the stated application use case, then verify that writing to a BLE characteristic works without regressing reads; the issue names no files or tests to run.

Written by the indexing model from the issue text.