add PKCE support

[aaronpk]

enable with pkce: true when creating the strategy.

Closes #83 and #56

[kamronbatman]

Great job, thank you!!

[darknightdev]

Great job! @kamronbatman We need this feature as well, can we please merge and publish? Thanks

[kamronbatman]

Waiting on @jaredhanson to validate, add tests, and determine if there are any breaking changes. It sounds silly but many people rely on this. If I get time next week I'll work with him on tests.

[MattPaymentus]

I tested this further. The call back on the backend doesn't pass the code challenge with it so it errors out.

[aaronpk]

@MattPaymentus not sure what you mean, I have this code working against an authorization server that requires PKCE.

[quichef]

Hello, we'd be interested on my side with this PR

[Hugo-Dlg]

Hi @jaredhanson and @kamronbatman, any update on it ? Thanks

[VVlasy]

Is the project maintainer unavailable?

[jackofsometrade]

Community is still waiting on support for PKCE support, FWIW...

[mdelnegro-lennd]

Any update?

[aaronpk]

@jaredhanson it'd be great to get this pushed out!