Enable custom converters for single fields

[sothawo]

The standard converters for Spring / Spring Data allow conversion between a class T and a String in Spring Data Elasticsearch. It is not possible to define a converter only for a single property of an entity. This might be handy, when for example a String property should be converted into a different value before storing and reconverted after retrieving a document.

Spring Data Elasticsearch already has the possibility for converters attached to a property (conversion for date fields or range properties), this should be extended to allow for custom defined converters, either by adding an attribute to @Field or by introducing a new annotation.