Fix potential jetcd core shading problem

[Shawyeok]

Motivation

There is a potential jar shading issue introduced in apache#4426 that causes NoClassDefFoundError when connecting to an etcd metadata store.

The jetcd-core-shaded module was introduced in apache#4426 to address the compatibility issues between jetcd-core’s grpc-java dependency and Netty. You can find more details here and in the grpc-java documentation.

Currently, we use unpack-shaded-jar execution unpacks the shaded jar produced by maven-shade-plugin:shade into the jetcd-core-shaded/target/classes directory. However, the classes in this directory conflict with its dependencies. If the maven-shade-plugin:shade runs again without cleaning this directory, it can produce an incorrect shaded jar. You can replicate and verify this issue with the following commands:

# Step 1: Clean the build directory
mvn clean

# Step 2: Perform an install and unpack the shaded jar into a directory.
# Verify the import statement for `io.netty.handler.logging.ByteBufFormat` in 
# `org/apache/pulsar/jetcd/shaded/io/vertx/core/net/NetClientOptions.class`. 
# The correct import should be: 
# `import io.grpc.netty.shaded.io.netty.handler.logging.ByteBufFormat;`.
mvn install
unzip $M2_REPO/org/apache/bookkeeper/metadata/drivers/jetcd-core-shaded/4.18.0-SNAPSHOT/jetcd-core-shaded-4.18.0-SNAPSHOT-shaded.jar \
  -d metadata-drivers/jetcd-core-shaded/target/first-classes

# Step 3: Run the install command again without cleaning.
# The unpacked jar from the previous step will persist in `target/classes`. 
# Unpack the shaded jar into a different directory (e.g., second-classes) and check the import.
# The incorrect import will be: 
# `import io.grpc.netty.shaded.io.grpc.netty.shaded.io.netty.handler.logging.ByteBufFormat;`.
mvn install
unzip $M2_REPO/org/apache/bookkeeper/metadata/drivers/jetcd-core-shaded/4.18.0-SNAPSHOT/jetcd-core-shaded-4.18.0-SNAPSHOT-shaded.jar \
  -d metadata-drivers/jetcd-core-shaded/target/second-classes

# Step 4: Use IntelliJ IDEA's "Compare Directories" tool to compare the `first-classes` 
# and `second-classes` directories. The differences in imports should become apparent.

We can remove the attach and unpack configurations, and it should work fine.

This issue has already been reported in apache/pulsar, and a similar patch has addressed it.