Add summary and description to OpenAPI definition for C++ endpoints

[letmaik]

The auto-generated OpenAPI doc lacks endpoint summaries and descriptions for C++ endpoints, and hence the docs lack them too. I think adding them would go a long way to help in understanding CCF's API.

Related to https://github.com/microsoft/CCF/issues/1918. The Azure guidelines only say there must be an OpenAPI definition, but doesn't say how detailed it should be.

For inspiration, https://github.com/Azure/azure-rest-api-specs contains OpenAPI 2.0 (Swagger) definitions for all (most?) Azure services, for example Service Fabric's data plane API.