[core-http] Find additionalProperties in referenced mapper

[joheredi]

When serializing object using mappers with references to other mappers, core-http doesn't look at the referenced mapper for additionalProperties it only looks for it in the source mapper.

Sample of a referenced mapper:

export const MonitorBase: CompositeMapper = {
  type: {
    name: "Composite",
    className: "MonitorBase",
    modelProperties: {
      baseType: {
        serializedName: "baseType",
        type: {
          name: "String"
        }
      },
      baseData: {
        serializedName: "baseData",
        type: {
          name: "Composite",
          className: "MonitorDomain" // this is a reference to MonitorDomainMapper
        }
      }
    }
  }
};

export const MonitorDomain: CompositeMapper = {
  type: {
    name: "Composite",
    className: "MonitorDomain",
    additionalProperties: { type: { name: "Object" } },
    modelProperties: {
      test: {
        serializedName: "test",
        type: {
          name: "String"
        }
      }
    }
  }
};

This PR modifies the serializer and deserializer so that they also look at the referenced mapper for additionalProperties and ports the changes to core-client

[xirzec]

Change looks good! Thanks for adding test coverage too! 👍

[bterlson]

Seems good to me.