Added a files endpoint, corrected a documents bug in answers requests, added error messages to API error handling (#6)

Co-authored-by: eyelevelai <33876565+eyelevelai@users.noreply.github.com>

files.go

@@ -0,0 +1,50 @@
+package gogpt
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+)
+
+// File struct represents an OpenAPI file
+type File struct {
+	Bytes     int    `json:"bytes"`
+	CreatedAt int    `json:"created_at"`
+	ID        string `json:"id"`
+	FileName  string `json:"filename"`
+	Object    string `json:"object"`
+	Owner     string `json:"owner"`
+	Purpose   string `json:"purpose"`
+}
+
+// FilesList is a list of files that belong to the user or organization
+type FilesList struct {
+	Files []File `json:"data"`
+}
+
+// ListFiles Lists the currently available files,
+// and provides basic information about each file such as the file name and purpose.
+func (c *Client) ListFiles(ctx context.Context) (files FilesList, err error) {
+	req, err := http.NewRequest("GET", c.fullURL("/files"), nil)
+	if err != nil {
+		return
+	}
+
+	req = req.WithContext(ctx)
+	err = c.sendRequest(req, &files)
+	return
+}
+
+// GetFile Retrieves a file instance, providing basic information about the file
+// such as the file name and purpose.
+func (c *Client) GetFile(ctx context.Context, fileID string) (file File, err error) {
+	urlSuffix := fmt.Sprintf("/files/%s", fileID)
+	req, err := http.NewRequest("GET", c.fullURL(urlSuffix), nil)
+	if err != nil {
+		return
+	}
+
+	req = req.WithContext(ctx)
+	err = c.sendRequest(req, &file)
+	return
+}