Allow specifying Assembly for migrations

[davidnajar]

    - Allow to specify a migrations assembly using an Assembly type apart from an assemlyName

Fixes #32461

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo

[davidnajar]

@dotnet-policy-service agree

[ajcvickers]

Thanks for your contribution!