ElementInfoManagerInterface must extend DiscoveryInterface

Problem/Motivation

This soft-blocks #2856950: Add a possibility to disable inline form errors for a complete form.

ElementInfoManagerInterface, unlike other plugin manager interfaces, does not extend anything.
Usually other plugin manager interfaces extend PluginManagerInterface, which in turn extends DiscoveryInterface, FactoryInterface, and MapperInterface.

Only DiscoveryInterface is relevant to ElementInfoManagerInterface, since the plugins should not be instantiated by outside code, only ::getInfo() and ::getInfoProperty() should be used.

However, implementations of hook_element_info_alter() may need to check the definitions of the element info. This would allow them to do so.

Proposed resolution

Make ElementInfoManagerInterface extend DiscoveryInterface

Remaining tasks

N/A

User interface changes

N/A

API changes

Strictly speaking yes, but with no real risk since as a plugin manager it should expose these methods.
And ElementInfoManager already extends PluginManagerBase.

Data model changes

N/A