Introduce a FetchModeTrait to allow emulating PDO fetch modes

Problem/Motivation

From #3265086: Implement statement classes using \Iterator to fix memory usage regression and prevent rewinding.

StatementPrefetch::fetch() has some code that allows emulating different PDO fetch modes, since all data is prefetched on execution in FETCH_ASSOC format, and needs to be re-formatted according to the fetch mode used in each fetch. This code could be reused for contrib drivers that are not based on PDO.

Also, the initialization of a FETCH_CLASS fetch object with constructor arguments is based on reflection - since PHP 5.6 we could have been using the splat operator (...) instead.

Proposed resolution

• Extract the fetch emulation code in a trait with appropriate protected methods that can be used in statement classes.
• Implement that in the StatementPrefetch.

Remaining tasks

User interface changes

none

API changes

A new trait with protected methods.

Data model changes

none

Release notes snippet