Add support for TFT display ST7796S

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

Add support for the TFT display ST7796S

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

First attempts at using the ST7789V display driver didn't work and two things of note came up.
* This (all?) display drivers require a reset pin, but in some cases (TTGO-T-WATCH) this isn't wired up, maybe as is usually the case in the ESP-IDF, accept `-1` to mean not in use
* Display drivers seem to hard code dimensions, making this more flexible may help with some of these very similar but not quite displays.

**Additional context**

Start by reviewing the existing ST7789V display-driver support and how reset pins and display dimensions are handled. Done means ST7796S support is available, including the requested handling for an unused reset pin and less rigid dimensions, with appropriate verification in the project’s existing display tests.

Written by the indexing model from the issue text.