Add assignment grades CSV download command

[zrdaley]

Fixes https://github.com/github/gh-classroom/issues/5

This command will be available for use with the following flags:

gh classroom assignment-grades -a 4876 -f $download-file-path

Pre-requisites:

• [x] Ability to query grades API endpoint

Previous implementation: https://github.com/github/gh-classroom/pull/22

[zrdaley]

I was able to confirm that the command is working in my codespace.

Grades exist case

@zrdaley ➜ /workspaces/gh-classroom (zrdaley/assignment_grades) $ go run main.go assignment-grades
? Select a classroom: zrdaleytestorg-classroom-02aa72
? Select an assignment: test
@zrdaley ➜ /workspaces/gh-classroom (zrdaley/assignment_grades) $ cat grades.csv 
assignment_name,assignment_url,starter_code_url,github_username,roster_identifier,student_repository_name,student_repository_url,submission_timestamp,points_awarded,points_available
test,https://classroom.github.com/classrooms/126513340-zrdaleytestorg-classroom-02aa72/assignments/test,,zrdaley,,test-zrdaley,https://github.com/zrdaleytestorg/test-zrdaley,,0,0

^ This was the same as the CSV exported through the UI.

zrdaley@Zenaras-MBP Desktop % cat test-grades-1692821024.csv 
"assignment_name","assignment_url","starter_code_url","github_username","roster_identifier","student_repository_name","student_repository_url","submission_timestamp","points_awarded","points_available"
"test","https://classroom.github.com/classrooms/126513340-zrdaleytestorg-classroom-02aa72/assignments/test","","zrdaley","","test-zrdaley","https://github.com/zrdaleytestorg/test-zrdaley","","0","0"

Both files open in the Numbers mac app.

No grades exist case:

@zrdaley ➜ /workspaces/gh-classroom (zrdaley/assignment_grades) $ go run main.go assignment-grades
? Select a classroom: zrdaleytestorg-classroom-02aa72
? Select an assignment: test
2023/08/23 19:53:39 No grades were returned for assignment
exit status 1