Introduce MemCS per-column index layout

[TarantoolBot]

Product: Tarantool EE
Since: 3.6

Now one can specify a per-column layout within a MemCS index
via space format definition and/or secondary index definition covers
option. So currently we have 3 places where an index layout can be
specified. Let's list them in order of precedence (from highest precedence
to lowest):

1. Within an index definition's covers section:

box.space.test:create_index('sk', {
    parts = {'c2', 'c3'},
    covers = {{'c4', layout = 'plain'}, {'c5',  layout = 'null_rle' }},
})

2. Within an index definition's layout section, which specifies
   the default layout for a non-key nullable field:

box.space.test:create_index('sk', {
    parts = {'c2', 'c3'},
    covers = {'c4', 'c5'},
    layout = 'null_rle'
})

3. Within a space format's field definition:

box.space.test:format({
    {name = "c2", type = "number"},
    {name = "c3", type = "number"},
    {name = "c4", type = "number", is_nullable = true, layout = 'null_rle'},
    {name = "c5", type = "number", is_nullable = true},
})

Currently the only available layouts are 'plain' and 'null_rle'.
Requested by @pianoyeg94 in https://github.com/tarantool/tarantool-ee/commit/9b74d277c38784dcff6cdcce38afe4f04a83d4af.