Add support for Proof Key for Code Exchange

[jarrodek]

Add an UI to enable Proof Key for Code Exchange (PKCE).
On the UI level there should be an option to (on/off) to tell the application that the server requires PKCE for authentication. The @advanced-rest-client/oauth-authorization or a hosting application (depending on configuration) should include code challenge/verifier with the request.

PKCE is not to be a default option when making a request. It has to be and option to be selected by the user.

Resources:

• https://tools.ietf.org/html/rfc7636
• https://auth0.com/docs/flows/concepts/auth-code-pkce

This is related to: advanced-rest-client/arc-electron#221