Add support to migrate from gogs (#14342)

Add support to migrate gogs:

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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

vendor/github.com/gogs/go-gogs-client/repo_collaborator.go

@@ -0,0 +1,44 @@
+// Copyright 2016 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package gogs
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+)
+
+type Collaborator struct {
+	*User
+	Permissions Permission `json:"permissions"`
+}
+
+type AddCollaboratorOption struct {
+	Permission *string `json:"permission"`
+}
+
+func (c *Client) ListCollaborator(user, repo string) ([]*Collaborator, error) {
+	collabs := make([]*Collaborator, 0, 10)
+	return collabs, c.getParsedResponse("GET", fmt.Sprintf("/repos/%s/%s/collaborators", user, repo), nil, nil, &collabs)
+}
+
+func (c *Client) AddCollaborator(user, repo, collaborator string, opt AddCollaboratorOption) error {
+	body, err := json.Marshal(&opt)
+	if err != nil {
+		return err
+	}
+	_, err = c.getResponse("PUT", fmt.Sprintf("/repos/%s/%s/collaborators/%s", user, repo, collaborator), jsonHeader, bytes.NewReader(body))
+	return err
+}
+
+func (c *Client) DeleteCollaborator(user, repo, collaborator string) error {
+	_, err := c.getResponse("DELETE", fmt.Sprintf("/repos/%s/%s/collaborators/%s", user, repo, collaborator), nil, nil)
+	return err
+}
+
+func (c *Client) IsCollaborator(user, repo, collaborator string) error {
+	_, err := c.getResponse("GET", fmt.Sprintf("/repos/%s/%s/collaborators/%s", user, repo, collaborator), nil, nil)
+	return err
+}