Update to resolve errors from openapi generated client code

During the process of testing some of the OpenAPI generated client code a few errors were encountered.

This PR adds "nullable": true to the time fields (created_at, deleted_at, etc.) as the ZMC northbound API returns null values if none exist.

Additionally, I found that the beamwidth was specified as a "format": "float" in the OpenAPI specification, however https://gitlab.flux.utah.edu/openzms/zms-zmc/-/blob/main/pkg/store/model.go?ref_type=heads#L269 in the model it is an int. This was modified within the OpenAPI spec to be "format": "integer".