Metadata: Add Field Converters API

[chaiwa-berian]

In SILC Server 0.1.0 alongside transformers we should introduce field converters, while field transformers usually do minor changes to the data stream (name changes etc), field converters can completely change the output of the data. See example 3 field converters:

Name	Description
size	Gives sizes of strings (length) and collections, i.e. /api/silcgroups?fields=members::size
isEmpty	Is string or collection empty, i.e. /api/silcgroups?fields=members::isEmpty
isNotEmpty	Is string or collection not empty, i.e. /api/silcgroups?fields=members::isNotEmpty