Add WithToken context override for per-request authentication

[Its-donkey]

Summary

• Adds WithToken(ctx, *Token) to allow per-request token overrides via context, enabling concurrent requests with different user tokens through a single client instance.
• Follows the same context-override pattern as NoCacheContext, checking context token before falling back to the client-level AuthClient or TokenProvider.
• Includes 5 new tests covering override behaviour, nil fallback, concurrency safety, and empty context.

Motivation

Endpoints like Get Channel Followers require a user token scoped to each channel (moderator:read:followers). Without per-request token overrides, users must create N separate Client instances to make concurrent requests for N different channels. WithToken solves this cleanly:

for _, channel := range channels {
    go func(ch Channel) {
        ctx := helix.WithToken(ctx, ch.UserToken)
        followers, _ := client.GetChannelFollowers(ctx, &helix.GetChannelFollowersParams{
            BroadcasterID: ch.ID,
        })
    }(channel)
}

Changes

• helix/client.go: Added WithToken, tokenFromContext, tokenContextKey. Modified doOnceWithResponse token resolution order (context > authClient > tokenProvider).
• helix/client_test.go: 5 new tests.