pinning/http: Allow caller to supply HTTP client

[3456091]

I'd like the ability to specify the HTTP client to use. What API do we want? Something like the following?

func (c *Client) WithHTTPClient(hc *http.Client) *Client {
	c.client.GetConfig().HTTPClient = hc
	return c
}

[Jorropo]

Note to self: when that is added I should add otelhttp's tracing HTTP client so we can send traceparent.