Add shutdown and safe_mode button

[jsuanet]

What does this implement/fix?

New buttons for shutdown and safe_mode, can be used as replacement for shutdown and safe_mode switches.

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#1725

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

button:
  - platform: safe_mode
    name: my_device safe_mode button
  - platform: shutdown
    name: my_device shutdown button

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 @jsuanet,
Thanks for submitting this pull request! Can you add yourself as a codeowner for this integration? This way we can notify you if a bug report for this integration is reported.
In __init__.py of the integration, please add:

CODEOWNERS = ["@jsuanet"]

And run script/build_codeowners.py

_{^{(message by NeedsCodeownersLabel)}}

[probot-esphome]

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

[jesserockz]

LGTM

[oxan]

At some point we can also deprecate the old switches.

[oxan]

Thanks!