[Issue #23] Update opportunity routes

[widal001]

Summary

Updates the Routes.Opportunities interface to use templates.

• Fixes [Spec] Opportunity routes #23
• Time to review: 7 minutes

Changes proposed

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

• Adds a new CommonGrants.Responses namespace with templates and models for success and error responses.
• Updates the Opportunities interface in CommonGrants.Routes to include a template and to use these new response models.
• Adds a "CommonGrants Base API" service definition

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.

View the updated README for more information

Additional information

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