[cleanup] PIP-462: Remove Etcd metadata store backend

[merlimat]

Motivation

Etcd metadata store backend is not used in production Pulsar deployments, yet maintaining it imposes significant costs:

• Dependency burden: jetcd-core pulls in gRPC, Netty, Vert.x and requires a dedicated shading module
• Distribution size: Increases Pulsar tarball and Docker image size for all users
• Library maintenance: jetcd is not actively maintained
• Maintenance overhead: Must be kept in sync with every MetadataStore interface change

Both ZooKeeper and Oxia are fully supported, battle-tested backends. The community's future efforts are focused on Oxia. Maintaining a third, unused backend dilutes this focus.

Changes

This PR adds the PIP-462 proposal document describing the removal of:

• EtcdMetadataStore and EtcdSessionWatcher implementation classes
• jetcd-core-shaded shading module
• jetcd-core and jetcd-test dependencies
• EtcdMetadataStoreTest

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[Denovo1998]

@merlimat @StevenLuMT
If the support for Etcd is deleted, can it be migrated to the https://github.com/apache/pulsar-java-contrib library? Just like the previous pulsar-sql(pulsar-presto) module has also been migrated to other code libraries.

[BewareMyPower]

@Denovo1998 Yes. Feel free to do that. Since this implementation has not been modified for long time, it could be as simple as a copy-and-paste.

[merlimat]

@Denovo1998 @StevenLuMT Added a PR to move it to contrib repo apache/pulsar-java-contrib#26