Don't raise errors when using datetime objects in as_datetime Jinja function/filter

[TheFes]

Proposed change

The as_datetime function/filter currently errors when a datetime object is used as value. This can be very annoying when it is unclear if a value is already a datetime object, or a string. Attribute values of entities can be native types, but in case of datetime objects they are shown as isoformat datetime strings in developer tools > states.
Different (custom) integrations use this differently, eg some custom integration with energy price forecast attributes use datetime objects (Nordpool) and others use datetime isoformat string (ENTSO-E).

This PR adds support for datetime objects to as_datetime. In case a full datetime object is used as value, it will retrun the object without changes. In case a datetime.date is used as value it will add midnight (just like it would for a date string like "2024-01-29")

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: Update as_datetime in templating docs home-assistant.io#32424

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
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format 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

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[github-actions]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days.
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Thank you for your contribution!

[TheFes]

Still waiting for a review :)

[TheFes]

@MartinHjelmare
Sorry for the tag, but as you did the review on the PR on which I initially added this, but on your request split into only adding the default parameter, could you please have a look at this one as well?

The previous PR was #107229

[emontnemery]

Looks good, thanks @TheFes 👍

[emontnemery]

The PR can be merged when there's a PR updating the documentation, can you look into that @TheFes ?

[TheFes]

Docs added

[emontnemery]

Failing tests are unrelated