A new `encode_key_order` option for json encoder introduced

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 https://github.com/tarantool/tarantool/commit/8806b7b561d58d1cef79e6c9c723827f4ce67ed8.

No target documentation file is named; first locate the JSON encoder and persistent serializer reference in the Tarantool docs. Document encode_key_order for json.encode and json.new().cfg, including the provided ordering examples and the behavior of unspecified keys; done when both per-call and persistent configurations are covered.