PKCE Authentication

[rgammans]

I've written a PoC PKCE implementation. This works with my small use case, but is there any demand to get this tidied up and integrated into the rest of the library?

PoC Here: https://gist.github.com/rgammans/af01ec49c850334a91eae51c44d30583

[freakboy3742]

If there's an authentication mode that Xero supports, we'd definitely like to support it.

[jeremyherbert]

Thank you! This works great!

[rgammans]

What do you need to be included in a PR for this? Currently, I've not got any tests, and there is no final 'success' page presented to the browser.

[freakboy3742]

@rgammans Tests are definitely a requirement; documentation would also be an important piece - especially if the implementation is missing an important "final step" that the user will need to implement.

[rgammans]

I'll add this to my TODO list, but I've got some unrelated work that is going to have to take priority.