New labels in config schema

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#9809

Product: Tarantool
Since: 3.1
Root document:

• https://www.tarantool.io/en/doc/latest/concepts/configuration/
• https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/
• https://www.tarantool.io/en/doc/latest/release/3.1.0/#experimental-connpool-module
• Mongodb docs for tags: https://www.mongodb.com/docs/manual/tutorial/configure-replica-set-tag-sets/

SME: @ Lord-KA

Details

The new labels are basically maps with string keys and values,
that are merged down to instance level. It means that if a replicaset
and an instance inside it have label with same key and different
value, the actual value is the one instance provided, e.g.

groups:
  group-001:
    replicasets:
      replicaset-001:
        labels:
          foo: 'true'
          bar: 'true'
        instances:
          instance-001:
            labels:
              baz: 'true'
              foo: 'false'

Results in:

./instance-001.iproto> require('config'):get('labels')
---
- baz: 'true'
  foo: 'false'
  bar: 'true'
...

Requested by @ Lord-KA in tarantool/tarantool@6d46a04.