[client] Expose all coordinator servers (leader and standbys) via the client API

[MaheshS08]

Purpose

Linked issue: close #4117

Brief change log

• Introduced a CoordinatorRole enum (LEADER / STANDBY) to distinguish coordinator roles.
• Extended PbCoordinatorServerInfo in the metadata proto to carry coordinator role and liveness status, and included this in MetadataResponse.
• Updated server-side metadata assembly to read all registered coordinators from ZooKeeper (/coordinators/ids), cross-referencing the active leader path (/coordinators/active) to assign roles.
• Added a liveness check for standby coordinators based on their presence/registration in the leader election group.
• Added a new Admin#describeCoordinators() API that returns all coordinators (leader and standbys) with their role and liveness status.

Tests

• Modified ZookeeperClientTest

API and Format

• Public API addition: New Admin#describeCoordinators() method returning the full coordinator HA topology (leader + standbys, with role and liveness). This is additive — no existing API signatures change.
• Wire format change: MetadataResponse / PbCoordinatorServerInfo gains new fields (coordinator role, liveness). Backward compatible — new fields are additive, and older clients/servers can ignore them.

[MaheshS08]

@affo , could you please review this PR?