FEATURE: add paravirtualized clock support for guest time access

[simongdavies]

Hyperlight guests can now read time without expensive VM exits by using a paravirtualized clock shared between host and guest. This enables high-frequency timing operations like benchmarking, rate limiting, and timestamping with minimal overhead.

Paravirtualized clocks work by having the hypervisor populate a shared memory page with clock calibration data. The guest reads this data along with the CPU's TSC to compute the current time entirely in userspace, avoiding the cost of a VM exit.

Reference: https://docs.kernel.org/virt/kvm/x86/msr.html#pvclock

The implementation uses the native mechanism for each hypervisor:

• KVM: pvclock (MSR 0x4b564d01)
• MSHV: Hyper-V Reference TSC page
• WHP: Hyper-V Reference TSC page

Guests have access to:

• Monotonic time: nanoseconds since sandbox creation, guaranteed to never go backwards
• Wall-clock time: UTC nanoseconds since Unix epoch
• Local time: wall-clock adjusted for host timezone captured at sandbox creation

Rust API (hyperlight_guest_bin::time):

• SystemTime/Instant types mirroring std::time
• DateTime type for human-readable date/time formatting
• Weekday/Month enums with name() and short_name() methods

C API (hyperlight_guest_capi):

• POSIX-compatible: clock_gettime, gettimeofday, time
• Broken-down time: gmtime_r, localtime_r, mktime, timegm
• Formatting: strftime with common format specifiers

The feature is gated behind guest_time (enabled by default) and documented in docs/guest-time.md.

Note: The timezone offset is a snapshot from sandbox creation and does not update for DST transitions during the sandbox lifetime.