cluster: provide `cbuilder` config modifying API right inside `cluster`

[Totktonada]

Let's hold config value inside cluster and allow to modify it with the cbuilder API.

This is convenient to don't write code like this in a test:

local config_1 = cbuilder:new(base_config)
     :set_glogal_option(<...>)
     :config()
local cluster = cluster:new(config_1)

-- <..do workload..>

local config_2 = cbuilder:new(config_1)
     :set_glogal_option(<...>)
     :config()
cluster:sync(config_2)

-- <..do workload..>

It is easy to take base_config instead of config_1 in the second cbuilder:new call by a mistake, for example.

I would like if the code above would look so:

local cluster = cluster:new(cbuilder:new(base_config)
     :set_glogal_option(<...>)
     :config())

-- <..do workload..>

cluster:sync(cbuilder:new(cluster:config())
     :set_glogal_option(<...>)
     :config())

-- <..do workload..>

Or, better:

local cluster = cluster:new(cbuilder:new(base_config)
     :set_glogal_option(<...>)
     :config())

-- <..do workload..>

cluster:modify_config()        -- returns cbuilder object initialized with cluster:config()
     :set_glogal_option(<...>) -- and saves all the modifications
cluster:sync() -- applies the previously created config

-- <..do workload..>

Naming is just what appears first in the head, let's discuss it.