support for UTF-8 (string) types with DocValuesType.BINARY

The goal is to add support for string fields with arbitrarily large values in the /export handler and streaming expressions.

StrField values are currently limited to 32766 bytes for the case where indexed=true or docValues=true. Exceeding this value triggers an "immense field" warning, and causes indexing to fail for the associated input doc.

Configuring a StrField field as "indexed=false docValues=false" removes this size limitation, so it is already possible to have large stored StrField values. But the "docValues=true" prerequisite for the /export handler (and consequently for streaming expressions) limits the size of field that can be used in conjunction with these features.

Adding support for UTF-8/string field types with DocValuesType.BINARY would address this limitation and allow considerable flexibility in the implementation of custom field types. N.b.: this would address field value retrieval use cases only (e.g., /export and useDocValuesAsStored); neither sorting nor faceting would be supported.