Adds retrieve payment methods customer endpoint

[macaugh]

This PR introduces a new request handler to simulate retrieving a payment method for a customer in the Stripe mock environment. The new handler is registered via the Customers.included callback using the route:

get /v1/customers/([^/]+)/payment_methods/([^/]+)

The corresponding method, retrieve_payment_method, extracts the customer ID and payment method ID from the URL, validates the existence of the customer, and then searches for the payment method within the customer's stored sources. If the payment method is not found, it raises an appropriate error.

I found based on the docs that to test you need to run the following:

$ bundle install
$ bundle exec rspec

However this led to a lot of errors.
To test the specific functions added you can use the following

bundle exec rspec spec/instance_spec.rb

[macaugh]

Still trying to decide our best option here. I'm thinking that this would work for us for now but I don't think it would be appropriate to upstream.

[hhoopes]

It seems like a solid first pass for improving our ability to use this gem! Will be interesting to get to upgrade the gem and start removing manual mocks from our tests.