Substitutions in !include variables

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

Ability to use substitutions in ```!include``` variables, like:
```yaml
packages:
output1: !include { file: common/package/output_gpio.yaml, vars: { id: 1, pin: GPIO15, type: ${channel_1_type}, inverted: False } }
```

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

**Additional context**

The request names ESPHome's YAML !include variables and provides an example, but no source file, test, or entry point. Start by locating the implementation and tests for !include and substitutions, then determine the supported syntax and add coverage showing that channel_1_type is resolved inside the vars mapping.

Written by the indexing model from the issue text.