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lua: add `encode_key_order` option to json encoder

This option allows to pass an array of keys that will be used to sort
keys in maps during serialization to json. The order of the provided keys
in the resulting string will be the same as in the `encode_key_order`
array. Keys, present in maps and not included in `encode_key_order` will
be serialized after the sorted ones in arbitrary order.

Closes #10606

@TarantoolBot document
Title: 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.

``` lua
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:
``` lua
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}'
```

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