[Context]: add ICorrelationIdGenerator with TimeProvider support

[rian-be]

Summary

Add a small, reusable ICorrelationIdGenerator that produces correlation identifiers, with TimeProvider injection for deterministic unit tests.

Goal

Give IContext implementations a clean supported way to mint CorrelationId (used by header propagation and logging) without each app reinvent the format.

Problem

Propagation examples hand-write X-Correlation-Id values ad hoc. There is no uniform generator, so formats differ across services, which breaks 1.1 header correlation and 1.2 trace correlation on the wire.

Scope

• ICorrelationIdGenerator abstraction.
• Default implementation (time + random/sequence based) supporting injected TimeProvider for deterministic tests.
• CorrelationContext sample that takes the generator and sets CorrelationId = generator.Next().
• Unit tests covering format, uniqueness, and TimeProvider-driven determinism.

Design Expectations

• Small, self-contained, composable (not a full propagation layer per 1.4).
• TimeProvider injection per the Category 6 determinism rule.
• Complements (does not replace) propagation — a generated id still must be carried by propagators (see [Context]: add distributed context propagation for cross-boundary correlation #10/[Context]: add W3C/B3/composite propagator adapters and queue carriers #12).

Acceptance Criteria

• Generator is injectable and produces reasonably unique, transport-friendly ids.
• Tests are deterministic when TimeProvider is substituted.
• No coupling to serialization, HTTP, or the core runtime.

Non-Goals

• No full propagation/header logic here — that lives in the propagator issues ([Context]: add distributed context propagation for cross-boundary correlation #10, [Context]: add W3C/B3/composite propagator adapters and queue carriers #12).
• No single mandated ID convention; format is left to the generator contract.