[ili9xxx] Add 18bit mode selection and custom init sequence

[clydebarrow]

What does this implement/fix?

Add new options to the ili9xxx component.

• pixel_mode option to choose between 16 and 18 bit mode
• init_sequence option to specify custom inititialisation sequences
• Add CUSTOM model to allow supporting new displays without code changes, using init_sequence.
• Fix 18 bit mode drawing in draw_pixels_at()

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#3836

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

display:
  - platform: ili9xxx
    data_rate: 40MHz
    model: ili9481
    init_sequence:
      - [ 0xD0, 0x07, 0x42, 0x18]
      - [ 0xD1, 0x00, 0x07, 0x10]
      - [ 0xD2, 0x01, 0x02]
      - [ 0xC0, 0x10, 0x3B, 0x00, 0x02, 0x11]
      - [ 0xC5, 0x03]
      - [ 0xC8, 0x00, 0x32, 0x36, 0x45, 0x06, 0x16, 0x37, 0x75, 0x77, 0x54, 0x0C, 0x00]

    pixel_mode: 18bit
    invert_colors: true
# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @nielsnl68, mind taking a look at this pull request as it has been labeled with an integration (ili9xxx) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.21%. Comparing base (4d8b5ed) to head (ff02d99).
Report is 626 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6745      +/-   ##
==========================================
+ Coverage   53.70%   54.21%   +0.50%     
==========================================
  Files          50       50              
  Lines        9408     9594     +186     
  Branches     1654     1691      +37     
==========================================
+ Hits         5053     5201     +148     
- Misses       4056     4069      +13     
- Partials      299      324      +25     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.