Add sticky bucketing feature

[madhuchavva]

1. Add Sticky Bucketing to ensure users persist in their assigned variations.

use std::sync::Arc;
use growthbook_rust::sticky_bucket::InMemoryStickyBucketService;

let sticky_service = Arc::new(InMemoryStickyBucketService::new());

let client = GrowthBookClientBuilder::new()
    .api_url(api_url)
    .client_key(sdk_key)
    .sticky_bucket_service(sticky_service)
    .build()
    .await?;

2. Update test spec to 0.7.1 and skips savedGroups tests as it is unsupported in this version

All scenarios defined in `all_cases.json` for `stickyBucket` are verified:

- use fallbackAttribute when missing hashAttribute: (Fixed bug to ensure fallback is used during hashing if primary is missing).
- performs evaluation without sticky bucket: Standard bucketing when no sticky doc exists.
- evaluates based on stored sticky bucket: Respects persisted assignment.
- does not consume a sticky bucket not belonging to the user: Validates attribute matching.
- upgrades a sticky bucket doc...: Promotes sticky assignment to primary attribute.
- favors a sticky bucket doc based on hashAttribute...: Priority handling.
- resets sticky bucketing when the bucketVersion changes: Versioning support.
- stops test enrollment when... blocked: Version blocking logic.
- disables sticky bucketing when disabled by experiment: Flags support.