schema: `unique_items` annotation

Since Tarantool 3.4 the experimental.config.utils.schema module has
the unique_items annotation taken into account by the <schema object>:validate() method.

The annotation can be assigned to a schema node that represents an array
of strings. An attempt to assign it to another schema node leads to an
error on the schema object creation (schema.new()).

The <schema object>:validate() method now raises an error if the
unique_items annotation is assigned and truthly, but the corresponding
value (an array) contains the same string two or more times.

The schema.set() constructor (creates an array with enumerated values
without duplicates) now assigns the unique_items annotation to the
created schema node instead of defining its own validate annotation.

The schema.set() constructor now allows a user to define the
validate annotation for the new schema node.

Example 1 (use unique_items):

local schema = require('experimental.config.utils.schema')

local s = schema.new('myschema', schema.array({
    items = schema.scalar({type = 'string'}),
    unique_items = true,
}))

s:validate({'foo', 'bar'})
-- OK

s:validate({'foo', 'foo'})
-- error: [myschema] Values should be unique, but "foo" appears at least
-- twice

Example 2 (use the schema.set() constructor):

local schema = require('experimental.config.utils.schema')

local s = schema.new('myschema', schema.set({'foo', 'bar'}))

s:validate({'foo', 'bar'})
-- OK

s:validate({'foo', 'foo'})
-- error: [myschema] Values should be unique, but "foo" appears at least
-- twice

Example 3 (use the validate annotation):

local fun = require('fun')
local schema = require('experimental.config.utils.schema')

local s = schema.new('myschema', schema.set({
    'router',
    'storage',
    'rebalancer',
}, {
    validate = function(data, w)
        local kv = fun.iter(data):map(function(x) return x, true end):tomap()
        if kv.rebalancer and not kv.storage then
            w.error('rebalancer needs storage role enabled')
        end
    end,
}))

s:validate({'storage', 'rebalancer'})
-- OK

s:validate({'rebalancer'})
-- error: [myschema] rebalancer needs storage role enabled

Example 4 (unique_items on an unsupported type):

local schema = require('experimental.config.utils.schema')

local s = schema.new('myschema', schema.array({
    items = schema.scalar({type = 'integer'}),
    unique_items = true,
}))
-- error: [myschema] "unique_items" requires an array of strings, got an
-- array of integer items

Requested by @Totktonada in https://github.com/tarantool/tarantool/commit/62c864dc2da8ff63bc63dd0050bc34020ed4aaa1.

Locate the existing documentation for experimental.config.utils.schema and review the schema.new(), schema.array(), schema.set(), and validate() entry points. Document unique_items behavior, schema.set() validation options, and unsupported-type errors using the supplied examples; done means the Tarantool 3.4 behavior and examples are covered accurately.