feat: Add support for a custom `http.Client`

[erezrokah]

Thank you for this lovely project 👋

Would it be possible to add an option to configure the http.Client set in: https://github.com/tailscale/tailscale-client-go/blob/b40c082c85b730c5976bcf283ab99495673a667f/tailscale/client.go#L77

This is useful for implementing retries via https://github.com/hashicorp/go-retryablehttp