Mock server with OAuth support

[kanadgupta]

Hi there! This is a great example, thanks for putting it together.

I'd love if there was a mock server (similar to how Petstore has one) available to make actual requests against.

The Petstore server even has a mock endpoint for an Implicit OAuth flow, it'd be great if there was one for Train Travel and its Authorization Code-based OAuth flow (Okta has a great playground that demonstrates this).