feat: store information for failed block migrations

[navinkarkera]

Description

• Updates ModulestoreBlockMigration table to allow storing null values in target field for blocks that failed to migrate/import.
• Adds unsupported_reason field to store reason for failure.
• Add number of children blocks in failed block unsupported_reason field.
• Fixes issue with blocks like openassessment where url_name field is not included in its olx during serialization.

Useful information to include:

• Which edX user roles will this change impact? "Course Author"

Supporting information

• Placeholder for failed blocks? frontend-app-authoring#2527
• Private-ref: https://tasks.opencraft.com/browse/FAL-4269
• Related frontend PR: feat: placeholder blocks for failed import blocks frontend-app-authoring#2703

Testing instructions

See openedx/frontend-app-authoring#2703

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.
• If your database migration can't be rolled back easily.

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[ChrisChV]

Looks good 👍

• I tested this: I followed the testing instructions
• I read through the code and considered the security, stability and performance implications of the changes.
• I tested that the UI can be used with a keyboard only (tab order, keyboard controls).
• Includes tests for bugfixes and/or features added.
• Includes documentation