Adds refresh token, custom claims, and pkce without client secret support to cloudflare-go

[Saud9911]

Adds refresh token, custom claims, and pkce without client secret support to cloudflare-go

Description

docs: https://developers.cloudflare.com/api/operations/access-applications-list-access-applications

Has your change been tested?

unit tests

Screenshots (if appropriate):

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• [ x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

Originally posted by @ajholland in cloudflare/cloudflare-go#1981