Add support for Sub-Issues (#3258)

I have added features to manage sub-issues.

These features are developed by referring to the GitHub API
documentation, while making sure to follow the original code style and
direction.

The changes included in this PR are as follows:

**1. Sub-issue management features**
- Get sub-issues of an issue

![image](https://github.com/user-attachments/assets/01342e56-abe5-4803-84e4-d5ff7b70c2d6)
```python
## get sub issues
sub_issues = repo.get_issue(1).get_sub_issues()
for i, v in enumerate(sub_issues):
    print(i, v.title)
```
```bash
0 test
1 test-12
```
- Add a sub-issue to an issue
```python
issue_a = repo.get_issue(1)
issue_b = repo.create_issue(title="issue_b", body="...")
issue_a.add_sub_issue(sub_issue_id=issue_b.id)
```
- Remove a sub-issue from an issue
```python
issue_a = repo.get_issue(1)
sub_issues = issue_a.get_sub_issues()
issue_a.remove_sub_issue(sub_issues[0].id)
```
- Change the priority of sub-issues within an issue
```python
issue_a = repo.get_issue(1)
sub_issues = issue_a.get_sub_issues()

d = { i:v.id for i, v in enumerate(sub_issues)}
# 0 2893485819
# 1 2897252593

issue_a.reprioritize_sub_issue(sub_issue_id=d[0], after_id=d[1])

for i, v in enumerate(issue_a.get_sub_issues()):
    print(i, v) 
# 0 2897252593
# 1 2893485819

```

**2. Unit tests to verify sub-issue features**

Thank you!

**refer**
- #3178  
- https://github.com/cli/cli/issues/10378

---------

Co-authored-by: Um Changyong <changyong.um@sfa.co.kr>
Co-authored-by: Enrico Minack <github@enrico.minack.dev>