feat: add can_vote property to tier entity

Summary

Part of #330 (closed) - tiers implementation.

• New property in configuration of tier - can_vote. Indicates whether instances from tier could be considered as voter in raft leader election.

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• Follow-up doc issue is created: docs#101 (closed)