Add 'IsRequired' column to 'matrix_subjects' table

[coderabbitai]

In the context of PR #25 (#25), it was noted that the 'IsRequired' column is missing from the 'matrix_subjects' table. This issue is to track the addition of this column as discussed in the comments (#25 (comment)). This task was deferred to a follow-up PR as the current PR is already extensive. @marianysilva and @arianerocha have previously discussed this matter.