[serial_channel] Add new entity for serial channel emulation

[clydebarrow]

What does this implement/fix?

Adding a new entity type and a serial_channel component to expose serial ports via the
ESPHome embedded webserver.

See also esphome/esphome-webserver#182

Testing - all I have done so far is generate the v3/server_index_v3.h file and copy it to
the esphome code for building, using local: true in the webserver.

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)
• Developer breaking change (an API change that could break external components)
• Code quality improvements to existing code or addition of tests
• Other

Related issue or feature (if applicable):

• fixes

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

• esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx
• LN882x
• nRF52840

Example entry for config.yaml:

# Example config.yaml

serial_channel:
  name: "Debug Serial"
  uart_id: uart_bus
  buffer_size: 512
  on_data:
    - logger.log:
        format: "Received data: %s"
        args: ["x.c_str()"]


web_server:
  local: true
  version: 3

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.

[github-actions]

To use the changes in this PR:

# Clone the repository:
git clone https://github.com/clydebarrow/esphome
cd esphome

# Checkout the PR branch:
git fetch origin pull/134/head:serial-entity
git checkout serial-entity

# Install the development version:
script/setup

# Activate the development version:
source venv/bin/activate

Now you can run esphome as usual to test the changes in this PR.

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