Reworked the add_datatype system and made much of PDI use it

• renamed Datatype_template_func into Datatype_template_parser and added a Context parameter
• do not expose Datatype_template_parser outside of PDI::Context
• replaced Data_descriptor::creation_template by Data_descriptor::default_type
• removed Data_descriptor::config
• replaced Datatype_template::load by Context::datatype
• removed the specific handling of array & record and instead handle them like all other scalar types
• added a default_type getter
• global_context.h and data_descriptor_impl.h are now private implementation headers that can not be used in plugins implementation.