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/user_key.go

@@ -0,0 +1,49 @@
+// Copyright 2015 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"
+	"time"
+)
+
+type PublicKey struct {
+	ID      int64     `json:"id"`
+	Key     string    `json:"key"`
+	URL     string    `json:"url,omitempty"`
+	Title   string    `json:"title,omitempty"`
+	Created time.Time `json:"created_at,omitempty"`
+}
+
+func (c *Client) ListPublicKeys(user string) ([]*PublicKey, error) {
+	keys := make([]*PublicKey, 0, 10)
+	return keys, c.getParsedResponse("GET", fmt.Sprintf("/users/%s/keys", user), nil, nil, &keys)
+}
+
+func (c *Client) ListMyPublicKeys() ([]*PublicKey, error) {
+	keys := make([]*PublicKey, 0, 10)
+	return keys, c.getParsedResponse("GET", "/user/keys", nil, nil, &keys)
+}
+
+func (c *Client) GetPublicKey(keyID int64) (*PublicKey, error) {
+	key := new(PublicKey)
+	return key, c.getParsedResponse("GET", fmt.Sprintf("/user/keys/%d", keyID), nil, nil, &key)
+}
+
+func (c *Client) CreatePublicKey(opt CreateKeyOption) (*PublicKey, error) {
+	body, err := json.Marshal(&opt)
+	if err != nil {
+		return nil, err
+	}
+	key := new(PublicKey)
+	return key, c.getParsedResponse("POST", "/user/keys", jsonHeader, bytes.NewReader(body), key)
+}
+
+func (c *Client) DeletePublicKey(keyID int64) error {
+	_, err := c.getResponse("DELETE", fmt.Sprintf("/user/keys/%d", keyID), nil, nil)
+	return err
+}