Update schema.json to correctly describe the 'alter column' operation

[andrew-farries]

Update schema.json to correctly describe the 'alter column' operation.

The alter column operation is used to perform different alter column operations according to which fields are present.

The rules are:

• table and column are always required.
• Exactly one of name, type, check, unique, references, nullable must be present.
• If type, check, unique, references or nullable is present, then up and down must also be present.
• If name is present then neither up or down is allowed.

This PR updates schema.json to capture these dependencies.

This change causes churn in the test code as some fields that previously string have become *string.

Fixes #222

[andrew-farries]

I think we should add some tests around schema validation for the 'alter column' operation here too.

[andrew-farries]

I pushed another update to the schema - it fixes the problem where name could be specified alongside other operations like check, nullable etc.

[andrew-farries]

I've added some tests for JSON schema validation in #263

[SferaDev]

Spec also LGTM