feat: add courseware-access pipeline steps

This commit replaces most enterprise-specific logic serving the purpose
of conditionally blocking courseware access.

Enterprise steps added:
- EnterpriseStartDateAccessFailureStep supplies an enterprise-specific start-date error for enterprise learners.
- ActiveEnterpriseCheckStep denies access when the learner's active EnterpriseCustomer differs from the enrollment's customer.

Consent steps added:
- DataSharingConsentRedirectStep supplies a redirect URL to collect data sharing consent when it is required.
- DataSharingConsentCourseAccessStep denies access when consent is required.

ENT-11544