Convenience functions for spans and events

[Doohl]

Currently, the experience for instrumenting custom spans and events with the ADOT SDK on Swift is a little cubersome and code-heavy. It shouldn't be too much effort to ship convenience functions for instrumenting apps with custom telemetry.

For example, customers could have the following experience:

// Simple span
AwsOpenTelemetryAgent.span("load_user_profile") { span in
    let user = database.getUser(userId: userId)
    processUser(user)
}

// Span with attributes
AwsOpenTelemetryAgent.span("checkout", attributes: [
    "cart.items": .int(3),
    "cart.total": .double(49.99)
]) { span in
    processCheckout()
}

// Async span with return value
let products = try await AwsOpenTelemetryAgent.span("fetch_products") { span in
    let result = try await api.getProducts()
    span.setAttribute(key: "count", value: result.count)
    return result
}

// Simple event
AwsOpenTelemetryAgent.event("button_clicked")

// Event with details
AwsOpenTelemetryAgent.event(
    "purchase_completed",
    body: "User completed checkout",
    attributes: [
        "order.id": .string("12345"),
        "order.total": .double(99.99)
    ]
)

ADOT Android SDK has this already: https://github.com/aws-observability/aws-otel-android/blob/main/core/src/main/kotlin/software/amazon/opentelemetry/android/features/CustomSpanClient.kt