Add new tag-on interface, `ToStringConverter` for "Key Serializers" to implement

In order to support cases of "stringified" serialization, we need to add a new interface; call it `ToStringConverter`, which can be implemented by one or both of:

1. Value serializers
2. Key serializers

The idea came from

https://github.com/FasterXML/jackson-datatypes-collections/issues/135

(or rather, its serialization counterpart)

where it is necessary to support "Stringified" serialization format for structured datatype.

Similar needs probably exists for other dataformats like CSV, and perhaps Properties.

Note: for deserialization, `KeyDeserializer` already works: but unfortunately we never added matching `KeySerializer` (intead, `JsonSerializer` is used, just has to write `JsonToken.FIELD_NAME` instead of `JsonToken.VALUE_STRING`.