Data API JsonCodec: Base64-binary support for vectors

[tatu-at-datastax]

(note: follow up to #1492)

After basic "JSON number array" input, also need to add support for optimized variant for vectors: Base64-encoded "packed floats" binary, in which client:

1. First converts from float[] (or platform equivalent) into IEEE-754 packed (4 bytes per float) byte sequence
2. Encodes resulting byte sequence into padded "MIME" Base64 String
3. Encloses String into EJSON "$binary" wrapping -- same as blob values

and Data API will decode, unpack things, passes to CQL driver in structure it expects.

This issue DOES NOT change handling on return path: it will use "simple"/"plain" JSON Number array representation -- this because we do not know which representation was used for insert/update (unlike with Collections where we keep a copy in doc_json)

For further work we'd file another issue as necessary to allow requesting optimal representation in results.