#177 feature/Invoice: init, added Invoice related structs, implemented met… (#249)

* feature/Invoice: init, added Invoice related structs, implemented methods:GenerateInvoiceNumber,GetInvoiceDetails(by ID)

* feature/Invoice: added implemented endpoints to README along with Usage

Co-authored-by: Akshay Prabhakant <akshayprabhakant@Akshays-MacBook-Pro.local>

Author: akshayDev17

README.md

@@ -101,6 +101,11 @@
 * POST /v1/billing/subscriptions/:id/capture
 * POST /v1/billing/subscriptions/:id/suspend
 * GET /v1/billing/subscriptions/:id/transactions
+
+### Invoicing
+
+* POST /v2/invoicing/generate-next-invoice-number
+* GET /v2/invoicing/invoices/:id
 
 ## Missing endpoints
 
@@ -371,6 +376,27 @@ c.DeleteWebhook("WebhookID")
 c.ListWebhooks(paypal.AncorTypeApplication)
 ```
 
+### Generate Next Invoice Number
+```go
+// GenerateInvoiceNumber: generates the next invoice number that is available to the merchant.
+c.GenerateInvoiceNumber(ctx) // might return something like "0001" or "0010".
+```
+
+### Get Invoice Details by ID
+```go
+// the second argument is an ID, it should be valid
+invoice, err := c.GetInvoiceDetails(ctx, "INV2-XFXV-YW42-ZANU-4F33")
+```
+* for now, we are yet to implement the ShowAllInvoices endpoint, so use the following cURL request for the same(this gives you the list of invoice-IDs for this customer)
+    ```bash
+    curl -v -X GET https://api-m.sandbox.paypal.com/v2/invoicing/invoices?total_required=true \
+    -H "Content-Type: application/json" \
+    -H "Authorization: Bearer <Token>" 
+    ```
+
+* refer to the beginning of this Usage section for obtaining a Token.
+    
+
 ## How to Contribute
 
 * Fork a repository

invoicing.go

@@ -0,0 +1,38 @@
+package paypal
+
+import (
+	"context"
+	"fmt"
+)
+
+// GenerateInvoiceNumber: generates the next invoice number that is available to the merchant.
+// Endpoint: POST /v2/invoicing/generate-next-invoice-number
+func (c *Client) GenerateInvoiceNumber(ctx context.Context) (*InvoiceNumber, error) {
+
+	req, err := c.NewRequest(ctx, "POST", fmt.Sprintf("%s%s", c.APIBase, "/v2/invoicing/generate-next-invoice-number"), nil)
+	nextInvoiceNumber := &InvoiceNumber{}
+	if err != nil {
+		return nextInvoiceNumber, err
+	}
+
+	if err = c.SendWithAuth(req, nextInvoiceNumber); err != nil {
+		return nextInvoiceNumber, err
+	}
+
+	return nextInvoiceNumber, nil
+}
+
+// GetInvoiceDetails: show invoice details for a particular invoice by ID.
+// Endpoint: GET /v2/invoicing/invoices/{invoice_id}
+func (c *Client) GetInvoiceDetails(ctx context.Context, invoiceID string) (*Invoice, error) {
+	req, err := c.NewRequest(ctx, "GET", fmt.Sprintf("%s%s%s", c.APIBase, "/v2/invoicing/invoices/", invoiceID), nil)
+	invoice := &Invoice{}
+	if err != nil {
+		return invoice, err
+	}
+
+	if err = c.SendWithAuth(req, invoice); err != nil {
+		return invoice, err
+	}
+	return invoice, nil
+}