Refactor nested IDs into structs

[sebasslash]

Description

There are a few endpoints which use nested identifiers to identify a resource rather than using the external ID of that resource. As of now the Module Registry API seems to follow this type of pattern, e.g

POST /organizations/:organization_name/registry-modules/:registry_name/:namespace/:name/:provider/versions

In order to keep method signatures readable, I've refactored these nested URL params into structs which represent the set of identifiers used when referencing a resource. The example above would look something like:

type RegistryModuleID struct {
  Organization string
  RegistryName string
  Namespace string
  Name string
  Provider string
}

Not only does this help keep our method signatures compact, but it enables us to keep our validations DRY for more consistency.

[brandonc]

I like it. Let's update #333, too.