CMS-2043 Introduce InputTypeResolver for resolving both built-in and custom input types
Given a name InputTypeResolver resolves both built-in and custom input types.

Added method getInputType to InputTypeExtensions:
Could not retrieve a specific InputType from InputTypeExtensions.
Removed final keyword on add/remove extension points methods,
so that InputTypeExtension could maintain a map of each registered InputType by it's name.