extractor: use lf-value-json for JSON-encoding DAML values

[S11001001]

Extractor contains its own local JSON encoder (with no paired decoder), used to encode contracts for storage in database. It is mostly like lf-value-json, but for the following:

• Optionals were {"Some": ...} and {"None": {}}, as opposed to lf-value-json's arrays with null/unwrapped shortcut for the common unnested case.
• Maps were surrounded with {"Map": ...}, otherwise the same.

lf-value-json is intended to be the uniform, intuitive JSON format for all LF values, and we want all applications to shift to using that encoding. Moreover, it includes a paired decoder, and is tested to injectively encode LF values. So we can replace the current bespoke Circe encoder in extractor with an adaptation of the spray-json encoder from lf-value-json to Circe's interface.

To match current behavior, we'd set encodeDecimalAsString = true, encodeInt64AsString = false.

This is a prerequisite for using extractor's database storage for enhancing json-api with SQL-backed queries, i.e. #2781.