box: add SQL settings to _session_settings

Part of #4511

@TarantoolBot document
Title: _session_settings system space
The _session_settings system space used to view or change session
settings.

This space uses a new engine. This allows us to create tuples on
the fly when the get() or select() methods are called. This
engine does not support the insert(), replace(), and delete()
methods. The only way to change the setting value is update(),
which can only be used with the "=" operation.

Because space creates a tuple on the fly, it allows us to get a
tuple without saving it anywhere. But this means that every time
we get a tuple from this system space, it is a new tuple, even if
they look the same:

tarantool> s = box.space._session_settings
tarantool> name = 'sql_default_engine'
tarantool> s:get({name}) == s:get({name})
---
- false
...

Currently, this space contains only SQL settings, since the only
session settings are SQL settings.

List of currently available session settings:

sql_default_engine
sql_defer_foreign_keys
sql_full_column_names
sql_parser_debug
sql_recursive_triggers
sql_reverse_unordered_selects
sql_select_debug
sql_vdbe_debug

The default values of these settings cannot be changed by the
user.

Debug settings are disabled by default and can only be enabled in
the debug build.

Example of usage:
tarantool> s = box.space._session_settings
-- View session settings values.
tarantool> s:get({'sql_default_engine'})
---
- ['sql_default_engine', 'memtx']
...

tarantool> s:select()

s:select()
---
- - ['sql_default_engine', 'memtx']
  - ['sql_defer_foreign_keys', false]
  - ['sql_full_column_names', false]
  - ['sql_full_metadata', false]
  - ['sql_parser_debug', false]
  - ['sql_recursive_triggers', true]
  - ['sql_reverse_unordered_selects', false]
  - ['sql_select_debug', false]
  - ['sql_vdbe_debug', false]
...

tarantool> s:select('sql_g', {iterator='LE'})
---
- - ['sql_full_metadata', false]
  - ['sql_full_column_names', false]
  - ['sql_defer_foreign_keys', false]
  - ['sql_default_engine', 'memtx']
...

-- Change session setting value.
tarantool> s:update('sql_default_engine', {{'=', 'value', 'vinyl'}})
---
- ['sql_default_engine', 'vinyl']
...