feat: Sec. index on json (sourcenetwork#3330)

## Relevant issue(s)

Resolves sourcenetwork#2280

## Description

Enables json fields indexing.

JSON interface has been extended to allow traversing it with different
configurations.

Indexing or documents has been refactor so that instead of acting based
off of the fact that there is a special field in the index description
(like array or json), we assign a field-specific generator so that every
field is responsible for generating a value for the inde key. For
example, if we have a composite index made up of fields of types int,
array and json (complex composite index), we will generate all possible
combinations where int generator will always generate 1 value, array
generator will generate values for every element and json generator will
generate values for every json node.

Added json encoding/decoding to our encoding package.

Author: islamaliev

client/README.md

@@ -1,3 +1,5 @@
 The `client` package is the primary access point for interacting with an embedded DefraDB instance.
 
 [Data definition overview](./data_definition.md) - How the shape of documents are defined and grouped.
+
+[Secondary indexes](./secondary_indexes.md) - Using secondary indexes in DefraDB.