[2pt] Add example about working with varbinary fields

[akudiyar]

Product: Tarantool
Audience/target: developers
Root document: https://www.tarantool.io/en/doc/latest/book/box/data_model/
SME: ?

This might lead to restructuring the Indexes section, which is a separate task.

---

"How to save and load binary fields" is a frequently asked question by customers. There are no mentions of the right ways of using varbinary fields in the documentation.

An example of example:

tarantool> box.schema.create_space('test')
---
- engine: memtx
  before_replace: 'function: 0x010c252e98'
  on_replace: 'function: 0x010c252e60'
  ck_constraint: []
  field_count: 0
  temporary: false
  index: []
  is_local: false
  enabled: false
  name: test
  id: 512
- created
...
tarantool> box.space.test:create_index('primary', {unique=true, parts = {1, 'unsigned'}})
---
- unique: true
  parts:
  - type: unsigned
    is_nullable: false
    fieldno: 1
  id: 0
  space_id: 512
  type: TREE
  name: primary
...
tarantool> mp = require'msgpack'
---
...

tarantool> s = '\xc4\10aaaabbbbccccdddd'
---
...

tarantool> m = mp.encode(s)
---
...

tarantool> box.space.test:insert({1, m})
---
- [1, !!binary ssQKYWFhYWJiYmJjY2NjZGRkZA==]
...

tarantool> dg = require'digest'
---
...

tarantool> dg.base64_decode(mp.decode(box.space.test:select(1)[1][2]))
---
- !!binary aaaabbbbccccdddd
...

Also more information about using indexes with binary fields, limitations and typical patterns will be nice. The example above may need some beautification.