feat: AsyncEtcd v2

[fregataa]

refs #1772
related to lablup/etcetra#24

• Implement new AsyncEtcd client that uses etcetra.v2.client.EtcdCommunicator

TODO

• support automatic data migration from previous etcd client to this new etcd client. We could set it by config

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation

[achimnol]

This is superseded by #1870.