Feature: Let user configure/provide session for passthrough networking

[bstien]

Why?

Currently we're using a static/hardcoded URLSession for passthrough networking. Configuring this session should be allowed by the user, if they need to customize the URLSessionConfiguration.

This PR allows for each HTTPMock instance to have their own URLSession for passthrough networking.

What?

• Move passthroughSession to HTTPMock.
• Add methods to set/get sessions on HTTPMockURLProtocol.
• Add tests.
• Update README.

Show me

// Create a custom configuration for passthrough requests
let configuration = URLSessionConfiguration.default
let passthroughSession = URLSession(configuration: configuration)

// Pass it when creating HTTPMock instance
let httpMock = HTTPMock(passthroughSession: passthroughSession)
httpMock.unmockedPolicy = .passthrough

// Any unmocked requests will be sent via the provided `passthroughSession`