Fix primary key constraint

[ryannedolan]

Summary

Bug fix: Drop PRIMARY KEY constraint from KEY fields. Instead, support new PRIMARY_KEY "magic field" for the purpose.

Drop MySQL from integration tests, since the images involved do not support ARM.

Details

Flink SQL supports a PRIMARY KEY constraint, but not all KEYs can satisfy the constraint. In particular, keys that are nullable will cause problems.

In addition, not all connectors support primary keys -- even if they do support keys. For example, the kafka connector does not seem to support primary keys, whereas the upsert-kafka connector requires primary keys.

To workaround this issue, we drop the KEY "magic field" and instead introduce a PRIMARY_KEY "magic field". This enables adapters to use KEY or PRIMARY_KEY depending on the required semantics.

Testing

The constraint was manually removed from existing pipelines.