Add header with parsed date to imap event data

[jbouwh]

Proposed change

This PR adds date with the parsed email date as a datetime object to the imap custom event data. The date of an email message can be hard to parse as sometimes a timezone or comment is added. This helps to use the date in automations.
See also https://jrklein.com/2014/11/18/proper-mail-date-header-formatting-rfc-4021-rfc-2822-rfc-822-and-analysis-of-132k-date-headers/

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Add date header to imap custom event data home-assistant.io#26769

Checklist

• The code change is tested and works locally.
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• Tests have been added to verify that the new code works.

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[home-assistant]

Hey there @engrbm87, mind taking a look at this pull request as it has been labeled with an integration (imap) you are listed as a code owner for? Thanks!

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[frenck]

Thanks, @jbouwh 👍

../Frenck