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