[BUGFIX] PluginBase: Allow overwriting parts of configuration

Base-configuration comes from TypoScript. Allow to explicitly
provide only those values you want to override and keep the
rest unchanged.

Change-Id: Ie2efb9f78e6930de44e50d21846b7f8f7da1330e
Resolves: #54657
Releases: 4.0, 3.0
Reviewed-on: https://review.typo3.org/26585
Reviewed-by: Hans Höchtl
Tested-by: Hans Höchtl
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner