Shelly dimmer: Use unique_ptr to handle the lifetime of stm32_t

[edge90]

What does this implement/fix?

Automatic lifetime handling of stm32_init (stm32_t) via unique_ptr.

Quick description and explanation of changes

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#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

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

[martgras]

Looks good to me.
Only 2 minor format suggestions