Hostname customization

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

I'd like to customize wifi hostname. Now it's the same as device name, but it would be nice to choose a different one.

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

I have a lot of devices in my network configured with dhcp, and they appear on my router with the device name (eg: door)
I'd like to organize them with a prefix or with a custom hostname (eg: esphome-door or ha-door)
I tried changing device name but isn't a good solution.

**Additional context**

The issue names no files, tests, or entry points, so first locate the existing Wi-Fi hostname and device-name handling in the project. Done should mean users can choose a hostname independently of the device name, including a prefix or custom value, and that the chosen hostname appears in DHCP or router listings.

Written by the indexing model from the issue text.