Add config.storage_client documentation

This patch adds documentation for the `config.storage_client` module.
The module allows one to connect to a remote config.storage cluster and
use it as a key-value storage.

Examples for working both with Tarantool config.storage server API and
Tarantool config.storage client API are provided side by side since they
share quite the same API.

The examples on connecting to a config.storage instance via iproto has
been changed to an example on connecting to multiple config.storage
instances using the config.storage client.

Author: georgiy-belyanin

doc/code_snippets/snippets/centralized_config/instances.enabled/config_storage/myapp.lua

@@ -1,6 +1,84 @@
-net_box = require('net.box')
-local conn = net_box.connect('127.0.0.1:4401')
 local log = require('log')
-conn:watch('config.storage:/myapp/config/all', function(key, value)
-    log.info("Configuration stored by the '/myapp/config/all' key is changed")
-end)
+config = require('config')
+
+storage_client = config.storage_client.connect({
+    {
+        uri = '127.0.0.1:4401',
+        login = 'sampleuser',
+        password = '123456',
+    },
+    {
+        uri = '127.0.0.1:4402',
+        login = 'sampleuser',
+        password = '123456',
+    },
+    {
+        uri = '127.0.0.1:4403',
+        login = 'sampleuser',
+        password = '123456',
+    },
+})
+
+function register_watchers()
+    storage_client:watch('/myapp/config/all', function(_, revision)
+        log.info("Configuration stored by the '/myapp/config/all' key is " ..
+                 "changed. New revision number is %d.", revision)
+    end)
+end
+
+register_watchers()
+
+function connect_to_configured_storage()
+    -- `config.storage.endpoints` configuration section can be
+    -- passed directly as `connect()` options to connect to the
+    -- configured config.storage cluster.
+    local endpoints = config:get('config.storage.endpoints')
+    local storage_client = config.storage_client.connect(endpoints)
+
+    return storage_client
+end
+
+function put_config()
+    local fio = require('fio')
+    local cluster_config_handle = fio.open('../../source.yaml')
+    local cluster_config = cluster_config_handle:read()
+    local response = storage_client:put('/myapp/config/all', cluster_config)
+    cluster_config_handle:close()
+
+    return response
+end
+
+function get_config_by_path()
+    local response = storage_client:get('/myapp/config/all')
+
+    return response
+end
+
+function get_config_by_prefix()
+    local response = storage_client:get('/myapp/')
+
+    return response
+end
+
+function make_txn_request()
+    -- Execute an atomic request on the config.storage cluster.
+    local response = storage_client:txn({
+        predicates = { { 'value', '==', 'v0', '/myapp/config/all' } },
+        on_success = { { 'put', '/myapp/config/all', 'v1' } },
+        on_failure = { { 'get', '/myapp/config/all' } }
+    })
+
+    return response
+end
+
+function delete_config()
+    local response = storage_client:delete('/myapp/config/all')
+
+    return response
+end
+
+function delete_all_configs()
+    local response = storage_client:delete('/')
+
+    return response
+end

doc/platform/configuration/configuration_etcd.rst

@@ -260,6 +260,8 @@ In the example below, the following options are specified:
 -   ``timeout`` specifies the interval (in seconds) to perform the status check of a configuration storage.
 -   ``reconnect_after`` specifies how much time to wait (in seconds) before reconnecting to a configuration storage.
 
+You might use :ref:`config.storage_client <config_storage_client_api_reference>` API for connecting and controlling a remote config.storage cluster.
+
 You can find the full example here: `config_storage <https://github.com/tarantool/doc/tree/latest/doc/code_snippets/snippets/centralized_config/instances.enabled/config_storage>`_.