Add logging for assignment create

What does this MR do and why?

Resolves: #424556

Adds logging for user AddON assignment create action

• Adds info log when assignment is created successfully
• Adds info log if there were any validation error, such as 'No Seats Available'

How to set up and validate locally

All tests green.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.