box: support default field values in the space format

Now a field can be assigned a default value in the space format. When a new
tuple is inserted into a space, and some of the fields contain null values,
those fields will be filled with their respective default values.

Closes #8157

@TarantoolBot document
Title: Document default field values
Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/format/

The format clause contains, for each field, a definition within braces:
`{name='...',type='...'[,is_nullable=...][,default=...]}`, where:

* the optional `default` value contains a default value for the field.
  Its type must be compatible with the field type. If default value is set,
  it is applied regardless of whether `is_nullable` is true or false.

Example:

```lua
tarantool> box.space.tester:format{
         > {name = 'id', type = 'unsigned'},
         > {name = 'name', type = 'string', default = 'Noname'},
         > {name = 'pass', type = 'string'},
         > {name = 'shell', type = 'string', default = '/bin/sh'}}
---
...

tarantool> box.space.tester:insert{1000, nil, 'qwerty'}
---
- [1000, 'Noname', 'qwerty', '/bin/sh']
...
```