feat(v2/events/snowplow): add WithContext option to TrackEvent

What does this MR do and why?

Adds a WithContext functional option to Tracker.TrackEvent so callers can attach one or more Snowplow context entities (SelfDescribingJSON) alongside an event payload — for example, the gitlab_standard context, or any service-specific context.

Closes #103 (closed).

Note on CI: I haven't enabled instance runners on my fork (blocked on free-account identity verification, which I'd rather not pursue just for this MR). Labkit has ci_allow_fork_pipelines_to_run_in_parent_project: true, so a maintainer can run the pipeline in the parent project context with one click. Local verification on this branch is already green: cd v2 && go test -race ./events/snowplow/... passes, golangci-lint run ./events/snowplow/... reports 0 issues.

Why this design

• Functional options match existing labkit idioms (WithCallback, WithTokenSource, WithMetricsCollectorOptions) — no new public types other than the option function.
• Backward compatible: existing TrackEvent(name, props) calls compile and behave identically. No context is encoded when none is supplied. Satisfies the v2 stability principle ("never add required parameters").
• Encoding mirrors billing events: contexts are wrapped in a SchemaContexts envelope and base64-encoded into payload.ContextEncoded, the same path that TrackBillingEventWithOptionalInput already uses for the billable_usage context.
• Multiple contexts accumulate; nil entries are filtered.

Usage

gitlabStandard := &snowplow.SelfDescribingJSON{
    Schema: "iglu:com.gitlab/gitlab_standard/jsonschema/1-1-8",
    Data: map[string]any{
        "environment": "production",
        "source":      "artifact-registry",
    },
}

err := tracker.TrackEvent(
    "artifact_downloaded",
    map[string]any{"size_bytes": 1048576},
    snowplow.WithContext(gitlabStandard),
)

Changes

• New file v2/events/snowplow/tracker_options.go — defines TrackEventOption and the WithContext option.
• tracker.go — adds opts ...TrackEventOption to TrackEvent; encodes contexts when present, otherwise leaves ContextEncoded empty.
• tracker_test.go — adds TestTrackEvent_Context, a table-driven test with five subtests covering: no options, nil-only context filtered, single context, multiple contexts via one variadic call, and multiple WithContext calls accumulating.
• examples_test.go — adds ExampleTracker_TrackEvent_withContext demonstrating attaching a gitlab_standard context.

Public API surface added: WithContext and TrackEventOption. Both are documented.

How to test

cd v2 && go test -race ./events/snowplow/...

All existing tests still pass. New tests verify both the encoded-payload shape (the SchemaContexts envelope, contained schemas in declared order) and the no-context path (ContextEncoded stays empty).

golangci-lint run ./events/snowplow/... reports 0 issues.

Notes for reviewers

• I deliberately avoided changing TrackBillingEvent* — it already attaches a context via a different (non-optional) path. Unifying the two would be scope creep for this issue; happy to do a follow-up MR if preferred.
• Variadic opts ...TrackEventOption is preferred over a sibling TrackEventWithContext method because (a) it composes with future options without further API churn and (b) it mirrors NewEmitter(url, opts...) already in this package.