Consider deprecating old middleware API

**Feature request**
It would be great to have retry for core.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_retry` with default `false`.

Start by locating the core middleware and configuration handling in the terminal agent, then check whether retry behavior already exists. Done means the requested core retry behavior is defined and controlled by an `enable_retry` option whose default is `false`; clarify how this relates to the title's proposed deprecation before making changes.

Written by the indexing model from the issue text.