Support reloading the smtp notify platform

[bdraco]

Requires #39527

WTH: https://community.home-assistant.io/t/why-the-heck-is-a-restart-needed-for-each-an-every-change-to-configuration-yaml/219630/10?u=bdraco

• Frontend PR: Update reloadables to include telegram/smtp/mqtt frontend#6759
• Services.yaml:

Proposed change

Support reloading the smtp notify platform

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @fabaff, mind taking a look at this pull request as its been labeled with an integration (smtp) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}