Datetime implementations for From and Into u64

[Ctru14]

Add conversion implementations between Datetime and u64 by wrapping the to/from_unix_time_seconds() methods

[Copilot]

Pull request overview

This PR adds trait implementations to enable conversions between Datetime and u64 types using Rust's From and Into traits, wrapping the existing from_unix_time_seconds() and to_unix_time_seconds() methods.

Changes:

• Adds From<u64> for Datetime implementation for converting Unix timestamps to datetime objects
• Adds From<Datetime> for u64 implementation for converting datetime objects to Unix timestamps

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[williampMSFT]

This is a lossy operation, so I'm not sure if we want to allow this as an implicit operation - it truncates any sub-second component. Explicitly calling a 'to_seconds' method makes it pretty clear that this is happening, but if an into() can do it, that's pretty easy to do accidentally / miss at the call site.

We're not currently using sub-second precision on imxrt, but we could conceivably use it on other platforms for e.g. millisecond support on an ACPI time and alarm device. Why do we want to do this?

[Ctru14]

OpenDevicePartnership/embassy-imxrt#542 (comment) this option came up for a Datetime set API

[Ctru14]

But fair point, while one method could be made cleaner the tradeoff to lose clarity may not be worth it