Online DDL: switch unique key and column logic to declarative `schemadiff` analysis

[shlomi-noach]

vitess migrations require some analysis on participating columns (type, nullability, etc.) and of the table's unique keys (fiding an appropriate iteration key for the migration).

That analysis takes place today via information_schema:

https://github.com/vitessio/vitess/blob/1cc3e149508312a0ab01d95cbbe1cf996f0bbae6/go/vt/vttablet/onlineddl/schema.go#L456-L465

https://github.com/vitessio/vitess/blob/1cc3e149508312a0ab01d95cbbe1cf996f0bbae6/go/vt/vttablet/onlineddl/schema.go#L482-L545

https://github.com/vitessio/vitess/blob/1cc3e149508312a0ab01d95cbbe1cf996f0bbae6/go/vt/vttablet/onlineddl/schema.go#L550

https://github.com/vitessio/vitess/blob/1cc3e149508312a0ab01d95cbbe1cf996f0bbae6/go/vt/vttablet/onlineddl/schema.go#L558-L566

We want to move away from information_schema based analysis and into programmatic and declarative schemadiff analysis. We already ask schemadiff for instant-ddl capabilities and we generally want it to own as much of schema analysis as possible.

At the resolution of this issue, schemadiff should be able to tell, given two before and after tables, which unique keys ar ebest to use as iteration keys (if any) and what specific details we should know about the columns.