[TASK] Make Configuration default for ConfigurationInterface

[sbuerk]

It's a good practice to define interfaces and provide a default
implementation making xclass strategies, stubbing or mocking in
tests and similar techniques easier. Default implementation, if
provided, should be configured in the DI configuration as alias
for the interface, not done for the ConfigurationInterface on
the default implementation Configuration.

TYPO3 SingeltonInterface stems from old times before DI has
been introduced to TYPO3 and has been used to mark classes as
shared (only instanciated onces) within GeneralUtilitiy,
something is the default behaviour for services in DI as long
as not configured otherwise. Having ConfigurationInterface
and Configuration DI aware makes the SingeltonInterface
obsolete.

Services implementing SingletonInterface are automatically
configured as public: true by a TYPO3 DI compiler pass and
needs to be set manually when removing the interface from a
service.

This change ...

• Remove SingletonInterface from Configurtion in favour
  of default shared: true of the DI.
• Use Symfony AsAlias php attribute on Configuration to
  mark it as the default ConfigurationInterface service,
  addtional setting the service as public: true to allow
  retrieval using GeneralUtility::makeInstance().

That prepares towards retrieving services by DI either as
constructor or inject* methods in the future.