timeadd() function should accept "d" for days and possibly longer intervals.

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I've searched GH issues for all open/closed referencing "timeadd" and found nothing matching this request.

#### Description

The timeadd() function should allow longer intervals to be specified. It is possible to say "60 days" by converting that into hours but a comment has to be left to make it clear why "1440h" was specified.

#### Use Case(s)

My specific use case was the deprecate_at parameter for the aws EBS builder:
deprecate_at = timeadd(timestamp(), "1440h") # 60d * 24h = 1440

While this works I believe it would be more clear as to intent if the duration added could be in days or months.

#### Potential configuration

I would suggest "d" for days and "M" or "mo" for months. ("d" is unused at the moment and "mo" might be more clear than the month/minute conflict of M vs m)

#### Potential References

https://developer.hashicorp.com/packer/docs/templates/hcl_templates/functions/datetime/timeadd
https://developer.hashicorp.com/packer/integrations/hashicorp/amazon/latest/components/builder/ebs
(search for deprecate_at)

Start with the timeadd() documentation and implementation, then inspect the deprecate_at example in the AWS EBS builder. Clarify whether day support alone or month support is in scope, including the proposed syntax and month/minute ambiguity. Done means the accepted interval behavior and the referenced usage are clearly defined.

Written by the indexing model from the issue text.