In the home assistent is 8.8.8.8 hardcoded, is it posible to redirect to pihole ?
Second, is it possible with one click from the dashboard tot the prompt of the raspberry pi ? These features should be nice, what do you think about it ?
No. This need to be redirected at the router level.
Not in the Pi-hole software.
When 8.8.8.8 is hardcoded into the client, the DNS traffic from this client will never reach Pi-hole. Thus, Pi-hole has no ability to redirect anything.
As yubiuser noted, redirecting DNS traffic of this type requires software at the router level (or firewall). Typically, using this method, any direct query for 8.8.8.8 will be redirected to Pi-hole.