Enable adding columns in auto migrations in schema crate (#2956)

# Description of Changes

This adds auto migration support to the `schema` crate.
Additional changes are needed to fully implement this feature: see
#2912

# API and ABI breaking changes

None

# Expected complexity level and risk

1, at this level the change is fairly straightforward. The integration
will be more difficult.

# Testing

Added `schema` unit tests for success/failure cases.

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Co-authored-by: Shubham Mishra <[email protected]>

Author: kazimuth