feat(json): add option to encode decimals as string

Due to inconsistency of Tarantool type casting while using strict
data types as "double" or "unsigned" it is needed
to use "number" data type in a whole bunch of cases.
However "number" may contain "decimal" that will be serialized into
string by JSON builtin module.

This commit adds "encode_decimal_as_number" parameter to json.cfg{}.
That forces to encode `decimal` as JSON number to force type
consistency in JSON output.
Use with catious - most of JSON parsers assume that number is restricted
to float64.

NO_DOC=we do not host doc