Lazily init PulsarAdmin in PulsarAdminTool

[eolivelli]

Motivation

pulsar-admin (PulsarAdminTool) initialises eagerly the PulsarAdmin object and some of the the underlying REST API intefaces.
This initialisation process triggers lot of resource loading (like SSL/RESTAPI classes....) that slows down the JVM even for stuff that is not needed.
Also there are shutdown hooks that are useless by they are executed while existing from the command.

Removing initialisation of useless stuff helps in having a better bootstrap time, especially in case that you are not performing API calls, like when you are learning the tool and you make lots of syntax errors.

Modifications

• Initialise as lazily as possible PulsarAdmin
• Make PulsarAdminTool#main "testable" by allowing it to not call System.exit
• Use halt instead of exit in order to not trigger shutdown hooks

Verifying this change

This change is a trivial rework / code cleanup, but I have added tests for parts that have been touched and had not unit tests.

[wolfstudy]

Thanks @eolivelli work for this, please fix the License header error:

Error:  COMPILATION ERROR : 
Error:  /home/runner/work/pulsar/pulsar/pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdTopics.java:[1317,13] admin has private access in org.apache.pulsar.admin.cli.CmdBase
Error:  /home/runner/work/pulsar/pulsar/pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdNamespaces.java:[1109,13] admin has private access in org.apache.pulsar.admin.cli.CmdBase
Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project pulsar-client-tools: Compilation failure: Compilation failure: 
Error:  /home/runner/work/pulsar/pulsar/pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdTopics.java:[1317,13] admin has private access in org.apache.pulsar.admin.cli.CmdBase
Error:  /home/runner/work/pulsar/pulsar/pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdNamespaces.java:[1109,13] admin has private access in org.apache.pulsar.admin.cli.CmdBase

[eolivelli]

@wolfstudy thank you for the heads up, I have to rebase on master, probably some other changes went in.

[eolivelli]

@wolfstudy I have update this patch, thanks PTAL

[eolivelli]

This patch is good to go

[eolivelli]

I would be interested in sending a backport for 2.7
would it be accepted ?
I guess 2.7.x will stay for quite a log time, having a faster pulsar-admin would be cool

[sijie]

@eolivelli I would suggest leaving this change in master for now. We can backport it when it becomes a challenge when cherry-picking changes.

[eolivelli]

Good point. No need to pick it to 2.7