Go: Add `yaml` struct tags

[joeblubaugh]

Cog generates types with json struct tags:

type GeneratedStruct struct {
  FieldOne int `json:"fieldOne"`
}

Several popular Go yaml libraries need yaml tags set as well to serialize and deserialize correctly. It would be nice to add those to the generated code.

[joeblubaugh]

grafana-app-sdk is one of the primary users of cog, and I think many users will want to use YAML for their resources, since it's common practice for kubernetes-style applications to use YAML over JSON.