rust: time: Add HrTimerExpires trait

rust: time: Add HrTimerExpires trait

Introduce the `HrTimerExpires` trait to represent types that can be
used as expiration values for high-resolution timers. Define a
required method, `into_nanos()`, which returns the expiration time as a
raw nanosecond value suitable for use with C's hrtimer APIs.

Also extend the `HrTimerMode` to use the `HrTimerExpires` trait.

This refactoring is a preparation for enabling hrtimer code to work
uniformly with both absolute and relative expiration modes.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://lore.kernel.org/r/20250610132823.3457263-4-fujita.tomonori@gmail.com
[ changed conversion method names to `as_*` - Andreas ]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>