[Issue #22] Update data models

[widal001]

Summary

Updates the data models in the v0.1.0-alpha version of the CommonGrants TypeSpec library

• Fixes [Spec] Opportunity data model and base types #22
• Time to review: [xx] minutes

Changes proposed

What was added, updated, or removed in this PR.

• Adds the following models:
  • SystemMetadata which can be spread into other models to include createdAt and lastModifiedAt
  • OppTimeline which updates how key dates in the opportunity timeline are represented
  • CustomEnumValue which provides a flexible way of extending default enum values
• Reorganizes the models related to Opportunity
• Improves docstrings for the models in the package
• Updates Opportunity model in the following ways:
  • Removes templating -- that just added complexity
  • Adds a status property
  • Adds a keyDates property

Context for reviewers

Testing instructions, background context, more in-depth details of the implementation, and anything else you'd like to call out or ask reviewers. Explain how the changes were verified.

1. Checkout the PR
2. Run npm run docs to preview the updated library

Additional information

Screenshots, GIF demos, code examples or output to help show the changes working as expected.