Replace \PDO::FETCH_* constants to indicate fetch mode with an enumeration

Problem/Motivation

In #3345938: Deprecate support for unused \PDO::FETCH_* modes, we normalized the fetch modes used in Drupal's database operations. However, we fell short of introducing our own logic to indicate the fetch mode, and kept using \PDO::FETCH* constants for the purpose.

This issue is to define fetch modes independently from PDO so that non-PDO db libraries can implement their statement objects without needing to deal with constants that are not in their scope (thinking of the mysqli driver, mostly).

Steps to reproduce

Proposed resolution

• Introduce an enumeration to define Drupal's supported fetch modes
• Replace \PDO::FETCH_* constants with the enumeration where used
• Introduce a trait that introspects calls to PDO methods so that Drupal's statement objects can be abstract from the client library in use (and allow swapping them in contrib drivers)
• Deprecate usage of \PDO::FETCH_* constants in method calls outside of the above trait

Remaining tasks

User interface changes

Introduced terminology

API changes

The use of the \PDO::FETCH_* constants is deprecated and replaced with the new enum Drupal\Core\Database\Statement\FetchAs values:

• Drupal\Core\Database\Statement\FetchAs::Associative (\PDO::FETCH_ASSOC)
• Drupal\Core\Database\Statement\FetchAs::ClassObject (\PDO::FETCH_CLASS | \PDO::FETCH_PROPS_LATE)
• Drupal\Core\Database\Statement\FetchAs::Column (\PDO::FETCH_COLUMN)
• Drupal\Core\Database\Statement\FetchAs::List (\PDO::FETCH_NUM)
• Drupal\Core\Database\Statement\FetchAs::Object (\PDO::FETCH_OBJ)

Data model changes

Release notes snippet