Add method for get priority schema by project ID

[yongDataScince]

// PriorityScheme represents a scheme for same project
// The 'OptionIDs' field stores the priority IDs of a specific project
type PriorityScheme struct {
	ID              int      `json:"id"`
	Name            string   `json:"name"`
	Description     string   `json:"description"`
	DefaultOptionID string   `json:"defaultOptionId"`
	OptionIDs       []string `json:"optionIds"`
}

func (s PriorityService) GetPriorityScheme(ctx context.Context, projectID string) (*PriorityScheme, *Response, error) {
	apiEndpoint := fmt.Sprintf("rest/api/2/project/%s/priorityscheme", projectID)
	req, err := s.client.NewRequest(ctx, "GET", apiEndpoint, nil)
	if err != nil {
		return nil, nil, err
	}

	priorityScheme := new(PriorityScheme)
	resp, err := s.client.Do(req, priorityScheme)
	if err != nil {
		return nil, nil, NewJiraError(resp, err)
	}
	return priorityScheme, resp, nil
}