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

[marandaneto]

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.