[FFL-2891] Add configurable Flags initialization deadline

[sameerank]

What and why?

Add a configurable overall deadline for initializing DatadogFlags assignments.

The default five-second deadline prevents client initialization from waiting indefinitely when persisted storage or the assignment request stalls. Users can customize it through Flags.Configuration.initializationTimeout.

How?

• Start one deadline before reading cached assignments.
• Apply it across cache loading, assignment fetching, response decoding, and state updates.
• Cancel an in-flight assignment request when the deadline expires.
• Ignore cache and network results arriving after the deadline.
• Continue using matching cached assignments in the stale state; otherwise use coded defaults.
• Validate custom timeout values and fall back to five seconds for non-positive or non-finite values.
• Add coverage for cache delays, request cancellation, late responses, cached fallback, configuration, and API surface.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs - N/A; DatadogFlags has no Objective-C configuration surface
• Run make api-surface when adding new APIs

Validation completed:

• All 117 DatadogFlags tests pass in Swift 5 compatibility mode.
• Lint passes.
• API surface generation passes.

The default Swift 6 test build is currently blocked by an unrelated existing DatadogTrace OpenTelemetry Sendable compilation issue.