Introduce a convenience property list initializer, to align with OktaOAuth2 flows

[mikenachbaur-okta]

I discovered, while documenting the authentication flows, that InteractionCodeFlow didn't consume the default Okta.plist file, as in other authentication flows.

[emmanuelogunmuyiwa-okta]

Looks good!

[emmanuelogunmuyiwa-okta]

Since using the default plist now is an option, I think it might be beneficial to add a default/shared instance of InteractionCodeFlow and maybe we can update the Create the flow documentation as well to reflect this. Any thoughts on this ?

[mikenachbaur-okta]

The flows themselves are meant to be explicitly retained. The reason is that I want to avoid overuse / abuse of singletons. The WebAuthentication.shared singleton is an exception, since it's a common use-case. But using the flows directly is a more advanced use-case that is reasonable to assume a developer would be able to manage their memory properly.

[mikenachbaur-okta]

I like your point about updating docs to reflect this convenience. I'll do that before merging this PR.