PKCE support in the SDK

[nicolassanmar]

DocuSign now supports PKCE since March 2023 https://www.docusign.com/blog/developers/docusign-adds-support-pkce.
Unfortunately there is no way to make use of PKCE using the SDK, and I have not found workarounds.

We should add PKCE support to the SDK, and here are two different ways to do so:

1. Abstract away PKCE by adding an initialization option enablePKCE: true to the ApiClient constructor, where code_challenge and code_verifier are added to requests under the hood.
2. Add optional parameters to authentication functions to allow us to send our own code_challenge and code_verifier in each relevant call.

[ByungjaeChung]

DocuSign does not support Authorization Code Grant in our SDKs since it is webstack dependent. Instead, a generic (not DocuSign specific) OAuth library should always be used. Node for example is usually used with Passport. Some support PKCE, others do not.

[jmsrpp]

It would be acceptable for an additional optional parameter object to requestJWTUserToken (and ACG equivalent) where we can fill in the code_challenge and code_verifier.