feat(feature-flags): generalize driver-side cache

[cathleeny]

What type of PR is this?

• Refactor
• Feature

Description

Generalize the existing feature-flag cache for driver-owned flags on the Thrift path and before kernel/session initialization, once authenticated transport is available.

• Share raw flag values by workspace ID, with normalized host as a fallback; keep credentials and HTTP clients caller-owned.
• Add typed Boolean, int32, int64, double, string, and string-list getters. Integer validation uses standard-library fixed-width types.
• Replace the old generic getter with the Boolean getter in telemetry, preserving explicit opt-in/out behavior.
• Keep the blocking initial fetch and background refresh, retain stale values on refresh failures, and avoid queuing duplicate refreshes.

No kernel version bump, new kernel-selection flag, or automatic pre-kernel GET is included.

How is this tested?

• Unit tests
• Full non-real-kernel unit suite: 1,032 passed, 5 skipped, 1 deselected; 351 subtests passed.
• Focused telemetry/feature-flag suite: 58 passed.
• Black and mypy checks passed.
• Covers all six types, defaults/range validation, workspace sharing, and caller-owned refresh/authentication.

[cathleeny]

Replaced by #974, a direct upstream draft with identical commits. Closing this fork-based draft.