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.

Author: georgiy-belyanin

doc/code_snippets/snippets/centralized_config/instances.enabled/tarantool_config_storage/myapp_client.lua

@@ -0,0 +1,103 @@
+function put_config()
+    local fio = require('fio')
+    local cluster_config_handle = fio.open('../../source.yaml')
+    local cluster_config = cluster_config_handle:read()
+
+    -- Connect to a config.storage and put the config.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:put('/myapp/config/all', cluster_config)
+
+    -- Put a config to a configured config.storage cluster.
+    response = config.storage_client:put('/myapp/config/all', cluster_config)
+
+    cluster_config_handle:close()
+
+    return response
+end
+
+function get_config_by_path()
+    -- Connect to a config.storage and get /myapp/config/all.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:get('/myapp/config/all')
+
+    -- Get /myapp/config/all from a configured config.storage cluster.
+    response = config.storage_client:get('/myapp/config/all')
+    return response
+end
+
+function get_config_by_prefix()
+    -- Connect to a config.storage and get values with a /myapp/ prefix.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:get('/myapp/')
+
+    -- Get values prefixed /myapp/ from a configured config.storage cluster.
+    response = config.storage_client:get('/myapp/')
+    return response
+end
+
+function make_txn_request()
+    -- Connect to a config.storage and execute an atomic request.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:txn({
+        predicates = { { 'value', '==', 'v0', '/myapp/config/all' } },
+        on_success = { { 'put', '/myapp/config/all', 'v1' } },
+        on_failure = { { 'get', '/myapp/config/all' } }
+    })
+
+    -- Execute an atomic request on a configured config.storage cluster.
+    response = config.storage_client:txn({
+        predicates = { { 'value', '==', 'v0', '/myapp/config/all' } },
+        on_success = { { 'put', '/myapp/config/all', 'v1' } },
+        on_failure = { { 'get', '/myapp/config/all' } }
+    })
+
+
+    -- end-marker --
+
+    return response
+end
+
+function delete_config()
+    -- Connect to a config.storage and delete a key.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:delete('/myapp/config/all')
+
+    -- Delete a key on a configured config.storage cluster.
+    response = config.storage_client:delete('/myapp/config/all')
+
+    return response
+end
+
+function delete_all_configs()
+    -- Connect to a config.storage and delete all keys.
+    local storage_client = config.storage_client.connect({
+        '127.0.0.1:4401',
+        '127.0.0.1:4402',
+        '127.0.0.1:4403',
+    })
+    local response = storage_client:delete('/')
+
+    -- Delete all keys from a configured config.storage cluster.
+    response = config.storage_client:delete('/')
+    return response
+end