Replace array-based Schema API with a value object structure

Here's a MR that tries to do the minimum possible:

• add all the relevant value objects to the Drupal\Core\Database\SchemaDefinition namespace
• adds a converter from the new API to the old API + a basic test, and makes adjustments so that we can use the new API in hook_schema() or other functions without impacting the database driver code (that will be huge, but follow ups). In practice the new API is converted to the old one on the fly.
• added a couple of example conversions to demonstrate the new API, for system_schema() and Drupal\Core\Batch\BatchStorage::schemaDefinition(). I would leave conversions to followups, too, because it's a lot of work, especially for cases where the old array structure is built dynamically.

Looking forward to reviews!

Let’s change the trait to an abstract class.

For review. Invocation of hook_schema now return by default the array version of the schema definition, for BC. Code needs to explicitly opt-in to get Table objects instead or array shapes. In the future this will change but this allows to manage transition to the new approach.

Adding an enum for column size.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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Worth noting we're doing something similar but not quite the same over at 📌 Adding GIN and GIST indexes to PostgreSQL databases RTBC .

Per some feedback in Drupal Slack today, it sounds like the ideal state would be to use events a la ✨ Use events in Database Schema operations Active but we're postponed enough as it is on related issues. At the very least, this demonstrates how we can use objects in some circumstances inside of "ArrayPI" without breaking BC and enabling innovation.