Add a table to show relation between GitHub issue and PR

[Startrekzky]

Description

Add a table to show the relation between GitHub issue and PR, so that users can know the relation between these 2 entities.

Describe the solution you'd like

It's a domain layer table, the relation is transformed by issue events like the pic below

1. This issue is fixed by PR 31731
   
   

2. This issue is mentioned in PR 31753, 31754, 31755, etc.
   

3. Table design

• Table name: pull_request_issue
• 2 columns:
  • pull_request_id
  • issue_id

Describe alternatives you've considered

The relation between PR and issue is only for Github

[warren830]

After investigation, we can get all info of issues from issues/{number}/timeline:
https://api.github.com/repos/pingcap/tidb/issues/31467/timeline
In the response of this link, we can get event objects, and only the event object contains source.issue.pull_request are prs related to the issue, an example looks like the below(I deleted useless info):
{
"actor": {
"login": "ti-srebot",
"id": 66930949,
"node_id": "MDQ6VXNlcjY2OTMwOTQ5",
"avatar_url": "https://avatars.githubusercontent.com/u/66930949?v=4",
"gravatar_id": "",
"url": "https://api.github.com/users/ti-srebot",
"html_url": "https://github.com/ti-srebot",
"followers_url": "https://api.github.com/users/ti-srebot/followers",
"following_url": "https://api.github.com/users/ti-srebot/following{/other_user}",
"gists_url": "https://api.github.com/users/ti-srebot/gists{/gist_id}",
"starred_url": "https://api.github.com/users/ti-srebot/starred{/owner}{/repo}",
"subscriptions_url": "https://api.github.com/users/ti-srebot/subscriptions",
"organizations_url": "https://api.github.com/users/ti-srebot/orgs",
"repos_url": "https://api.github.com/users/ti-srebot/repos",
"events_url": "https://api.github.com/users/ti-srebot/events{/privacy}",
"received_events_url": "https://api.github.com/users/ti-srebot/received_events",
"type": "User",
"site_admin": false
},
"created_at": "2022-01-17T15:58:25Z",
"updated_at": "2022-01-17T15:58:25Z",
"source": {
"type": "issue",
"issue": {
"number": 31757,
"pull_request": {
"url": "https://api.github.com/repos/pingcap/tidb/pulls/31757",
"html_url": "pingcap/tidb#31757",
"diff_url": "https://github.com/pingcap/tidb/pull/31757.diff",
"patch_url": "https://github.com/pingcap/tidb/pull/31757.patch",
"merged_at": null
},
}
},
"event": "cross-referenced"
},