Add capture per-event context options (distinctId, groups, timestamp)

Feature request

Add Flutter SDK support for the mobile capture spec's optional per-event context fields:

• distinctId override
• groups context
• timestamp override

Why

The sdk-specs capture mobile variant includes optional per-event identity override, group context, and timestamp. These are useful for backfilled/offline/domain events where the event context differs from the SDK's current ambient user/session state.

Spec: https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/capture/spec.md

Current Flutter behavior

Posthog().capture() currently only accepts:

• eventName
• properties
• userProperties
• userPropertiesSetOnce

The platform interface also has a TODO noting missing capture parameters:

• posthog_flutter/lib/src/posthog.dart
• posthog_flutter/lib/src/posthog_flutter_platform_interface.dart

Desired outcome

Expose and forward the missing optional context fields through the Dart API and platform bridges, preserving existing behavior for callers that do not provide them.

Start with the capture API in posthog_flutter/lib/src/posthog.dart and the TODO in posthog_flutter/lib/src/posthog_flutter_platform_interface.dart. Trace how capture calls pass through the platform bridges, then add the optional fields while preserving existing calls without them. Verify that distinctId, groups, and timestamp reach the native implementations according to the linked capture spec.