API: Add an ContainerAutoGrader Entity

[jessehartloff]

An Autograder entity that stores the grading files and params for the grader.

The grader should also have a reference to the assignment that it autogrades. Details of the structure are still up for debate and discussion.

[jessehartloff]

After discussion: Create an entity named ContainerAutoGrader which will contain all information for autograding a submission inside a container. This will primarily be user to grade code submissions.

ContainerAutoGrader should have the following values (In addition to the common fields of id, createdAt, updatedAt, and deletedAt):

• assignmentID (The assignment to which this grader is attached)
• graderFilename (The filename for the grader tarball)
• makefileFilename (The filename for the grader Makefile. Can be null. If null, the default Makefile will be used)
• autogradingImage (String representing the Docker image to be used for grading)
• timeout (An int representing to max time for the grader in seconds)

A second NonContainerAutoGrader is planned for the future