Added a new is_sync parameter to `box.begin()`, `box.commit()`, and
`box.atomic()`. To make the transaction synchronous, set the `is_sync`
option to `true`. If any value other than `true/nil` is set, for example
`is_sync = "some string"`, then an error will be thrown.

Example:

```Lua
-- Sync transactions
box.atomic({is_sync = true}, function() ... end)

box.begin({is_sync = true}) ... box.commit({is_sync = true})

box.begin({is_sync = true}) ... box.commit()

box.begin() ... box.commit({is_sync = true})

-- Async transactions
box.atomic(function() ... end)

box.begin() ... box.commit()
```

Closes #8650

@TarantoolBot document
Title: box.atomic({is_sync = true})

Added the new `is_sync` parameter to `box.atomic()`. To make the
transaction synchronous, set the `is_sync` option to `true`. Setting
`is_sync = false` is prohibited. If to set any value other than true
for example `is_sync = "some string"`, then an error will be thrown.