Refactor Proto and dependent files

Cater to the following comments in the client query building code

```go
func buildSimplifiedListProductsRequest() *item.ProductListRequest {
return &item.ProductListRequest{
Offset: 1,
Limit: 2,
OrderBy: []*item.OrderByField{
{
Field: "category.name",
Direction: item.SortDirection_SORT_DESCENDING,
},
},
Where: map[string]*item.FilterCriteria{
"country_of_origin": { // it can be map. Take GraphQL schema and convert into proto
Operator: item.OperatorType_EQUALS,
Value: &item.FilterCriteria_StringValue{StringValue: "US"},
},
"category.name": {
Operator: item.OperatorType_EQUALS,
Value: &item.FilterCriteria_StringValue{StringValue: "T-Shirts"},
},
},
FieldMask: &fieldmaskpb.FieldMask{
Paths: []string{"id", "name", "price", "description", "manufacturer.name"}, // need clarity if manufacturer.name is the corrext syntax for field mask
},
NestedFilters: map[string]*item.NestedFilter{ // why do we need this?
NestedFilterReviews: {
Offset: 0,
Limit: 3,
OrderBy: []*item.OrderByField{
{
Field: "rating",
Direction: item.SortDirection_SORT_DESCENDING,
},
},
Where: map[string]*item.FilterCriteria{
"created_at": {
Operator: item.OperatorType_GREATER_THAN,
Value: &item.FilterCriteria_StringValue{StringValue: "2023-10-15T00:00:00Z"},
// There is no type safety here?
},
},`