Add Trigger Grow Fingerprint Reader on_scanning

**Describe the problem you have/What new integration you would like**
I would like to add a trigger for fingerprint readers that triggers when a finger is placed on the reader and verification is in progress.

**Please describe your use case for this integration and alternatives you've tried:**
I would like to add an indication to my system that the finger has been detected and is being verified. This way, on_finger_scan_matched or on_finger_scan_unmatched can be triggered afterwards to give more information about the laid finger.

**Additional context**
Having already been able to do this program in C++ Arduino, I know that it is possible to do this. Moreover, ESPHome displays in the logs that a finger is placed before having the result of the verification. So I think the integration is possible.

Start by locating the fingerprint-reader integration and the existing on_finger_scan_matched and on_finger_scan_unmatched entry points. Determine where the reader reports that a finger is placed and verification begins, then define on_scanning so it fires before either result event. Done means the new trigger reports the in-progress scan without changing the later matched or unmatched behavior.

Written by the indexing model from the issue text.