feat: Added enterprise subsidy events. OEP-49

[irfanuddinahmad]

Description: This PR adds OEP-49 data and signals for learner enrollment fee subsidies provided by enterprises.

Issue: Enterprises provide subsidies to their learners for enrollment. These subsidies are managed via enterprise related repositories such as enterprise-access, enterprise-subsidy, openedx-ledger. The subsidy redemption and reversal events would be produced/consumed in these repositories for enterprise budget adjustments.

Reference Usage https://github.com/openedx/enterprise-subsidy/pull/227 https://github.com/openedx/enterprise-access/pull/434

Reviewers:

• [x] tag reviewer
• [x] tag reviewer

Merge checklist:

• [x] All reviewers approved
• [x] CI build is green
• [x] Version bumped
• [x] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [x] Commits are squashed

Post merge:

• [x] Create a tag
• [x] Check new version is pushed to PyPI after tag-triggered build is finished.
• [x] Delete working branch (if not needed anymore)