Add PKCE flag to Identity Provider configuration

[GreenStage]

Adds PKCE to Identity Providers Configurations

Description

Some OAuth Identity providers support PKCE for confidential clients. This improves Cloudflare ZT Access overall strength against some attacks.

Has your change been tested?

Yes

Screenshots (if appropriate):

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• 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 (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[jacobbednarz]

LGTM with one question before we merge.