feat: make refund_entitlement pluggable

[syedsajjadkazmii]

This PR uses edx_django_utils.plugins to add pluggable_override to the refund_entitlement method for a customized implementation without affecting the open-source code.

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[aht007]

LGTM

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.