Make EnforcePKCE confurable

[damienbr]

Related issue

#1577

Proposed changes

Add in the oauth2.pkce.enforce part of the configuration file.

Checklist

• I have read the security policy
• I have read the contributing guidelines
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

Thank you!