A new encode_key_order option for json encoder introduced

[TarantoolBot]

Product: Tarantool
Since: 3.3.0

This option allows to specify the order in which the keys of object will be
serialized. To specify a particular order, it is possible to pass an
array of the keys, and resulting JSON string will contain the provided
keys in the specified order, while other present keys of an object will
be present after in some arbitrary order.

map = {a = 1, c = 100, b = 2}

require('json').encode(map)
-- returns '{"b":2,"a":1,"c":100}'

require('json').encode(map, {encode_key_order = {'a', 'b'}})
-- returns '{"a":1,"b":2,"c":100}'

require('json').(map, {encode_key_order = {'a', 'b', 'd'}})
-- returns '{"a":1,"b":2,"c":100}'

For a persistent serializer object a persistent ordering can be defined:

map_1 = {a = 1, c = 100, b = 2}

j = require('json').new()
j.cfg({encode_key_order = {'a', 'b'}})

j.encode(map_1)
-- returns '{"a":1,"b":2,"c":100}'

j.encode(map_1, {encode_key_order = {'c', 'b'}})
-- returns '{"c":100,"b":2,"a":1}'

j.encode(map_1)
-- returns '{"a":1,"b":2,"c":100}'

Requested by @grafin in tarantool/tarantool@8806b7b.