public final Class Mixin

Declares an API Interface to be included in this interface. The including
interface must redeclare all the methods from the included interface, but
documentation and options are inherited as follows:
- If after comment and whitespace stripping, the documentation
  string of the redeclared method is empty, it will be inherited
  from the original method.
- Each annotation belonging to the service config (http,
  visibility) which is not set in the redeclared method will be
  inherited.
- If an http annotation is inherited, the path pattern will be
  modified as follows. Any version prefix will be replaced by the
  version of the including interface plus the [root][] path if
  specified.
Example of a simple mixin:
    package google.acl.v1;
    service AccessControl {
      // Get the underlying ACL object.
      rpc GetAcl(GetAclRequest) returns (Acl) {
        option (google.api.http).get = "/v1/{resource=**}:getAcl";
      }
    }
    package google.storage.v2;
    service Storage {
      rpc GetAcl(GetAclRequest) returns (Acl);
      // Get a data record.
      rpc GetData(GetDataRequest) returns (Data) {
        option (google.api.http).get = "/v2/{resource=**}";
      }
    }
Example of a mixin configuration:
    apis:
    - name: google.storage.v2.Storage
      mixins:
      - name: google.acl.v1.AccessControl
The mixin construct implies that all methods in `AccessControl` are
also declared with same name and request/response types in
`Storage`. A documentation generator or annotation processor will
see the effective `Storage.GetAcl` method after inherting
documentation and annotations as follows:
    service Storage {
      // Get the underlying ACL object.
      rpc GetAcl(GetAclRequest) returns (Acl) {
        option (google.api.http).get = "/v2/{resource=**}:getAcl";
      }
      ...
    }
Note how the version in the path pattern changed from `v1` to `v2`.
If the `root` field in the mixin is specified, it should be a
relative path under which inherited HTTP paths are placed. Example:
    apis:
    - name: google.storage.v2.Storage
      mixins:
      - name: google.acl.v1.AccessControl
        root: acls
This implies the following inherited HTTP annotation:
    service Storage {
      // Get the underlying ACL object.
      rpc GetAcl(GetAclRequest) returns (Acl) {
        option (google.api.http).get = "/v2/acls/{resource=**}:getAcl";
      }
      ...
    }