Add set_reseed_interval to support tighter reseed cadences (#1523)

[Arul-Sujith]

This PR adds support for configuring a custom reseed interval in libcrux-hmac-drbg, directly addressing the feature request in issue #1523. It enables consumers to enforce a tighter, application-defined reseed cadence (e.g., reseeding from the OS every N bytes) as a defense-in-depth measure, independent of the DRBG's default global interval.

Changes

• Per-Instance Configuration: Expanded HmacDrbg state to maintain a local reseed_interval, which defaults to the SP 800-90A maximum (1 << 48).
• New Setters: Added pub fn set_reseed_interval(&mut self, interval: u64) to HmacDrbg and the rand-featured HmacDrbgRng wrapper.
• NIST Compliance (Clamping): The setter strictly bounds the input to the global RESEED_INTERVAL. A custom interval can force earlier reseeding, but it can never exceed the maximum interval allowed by the spec.
• Dynamic Logic: Updated the internal bounds checks in generate() and needs_reseed() to utilize self.reseed_interval.
• Hax Proofs: Updated the F* preconditions/postconditions to correctly target self.reseed_interval in place of the global constant, ensuring proofs remain sound.
• Testing: Added explicit test coverage (custom_reseed_interval and custom_reseed_interval_clamping) inside src/tests.rs to verify the early reseed triggering and the hard-cap logic.

Fixes #1523