Add EnableClientJsonOmitemptyTag to Config

[abe-tetsu]

EnableClientJsonOmitemptyTag is a boolean value that controls whether to add the omitempty option to JSON tags.

While gqlgen includes this as a configuration option, gqlgenc does not.
As a result, it has not been possible to control the presence of omitempty on the client side.

This PR introduces EnableClientJsonOmitemptyTag to address this limitation.

• Before the addition:

type Order struct {
	ID             string         "json:\"id\" graphql:\"id\""
	OrganizationID string         "json:\"organizationID\" graphql:\"organizationID\""
	LineItem       *OrderLineItem "json:\"orderLineItem,omitempty\" graphql:\"orderLineItem\""
}

• After the addition (when EnableClientJsonOmitemptyTag is set to false):

type Order struct {
	ID             string         "json:\"id\" graphql:\"id\""
	OrganizationID string         "json:\"organizationID\" graphql:\"organizationID\""
	LineItem       *OrderLineItem "json:\"orderLineItem\" graphql:\"orderLineItem\""
}