PCF8574 custom characters

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

I'd like to get an implementation of LiquidCrystal's method [createChar](https://www.arduino.cc/en/Reference/LiquidCrystalCreateChar).

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

I think it would be a nice addition to be able to define byte arrays for custom chars in display's definition. Maybe something like this:

```
- platform: lcd_pcf8574
dimensions: 16x2
address: 0x27
custom_characters:
- my_custom_char1:
- my_custom_char2: <...>
```

**Additional context**

Start by reviewing the ESPHome lcd_pcf8574 integration and the linked LiquidCrystal createChar reference. Determine how the proposed custom_characters configuration should represent byte arrays and how those definitions are sent to the display. Done means the integration accepts the configuration and supports custom characters as requested, with coverage for the new behavior.

Written by the indexing model from the issue text.