[Rejected] Add support for Java Extension Class Loader.

[wu-sheng]

• If this is non-trivial feature, paste the links/URLs to the design doc.
• Update the documentation to include this new feature.
• Tests(including UT, IT, E2E) are added to verify the new feature.
• If it's UI related, attach the screenshots below.
• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes #[Rejected] "java -ext.dirs" is not supported skywalking#3661.
• Update the CHANGES log.

To @apache/skywalking-committers

I am planning to bump up the version of Java agent after this gets merged. 9.0.0 would be the next major version to indicate we added Extension ClassLoader support in the core level.
First and most importantly, this credit and contribution belongs to @daimingzhi. I am just finishing this PR with a little conflict resolving, polishing, and fix a little error in the plugin test.

Extension ClassLoader

Extension ClassLoader: The Extension ClassLoader is a child of Bootstrap ClassLoader and loads the extensions of core java classes from the respective JDK Extension library.
It loads files from jre/lib/ext directory or any other directory pointed by the system property java.ext.dirs.
User decides which parts of the service should be loaded by the extension classloader, so, we open plugin.plugins_in_ext_class_loader=${SW_PLUGINS_IN_EXT_CLASS_LOADER:} in the agent.config file.

Through this setting, users declare plugins are for excClassloader. Multiple values should be separated by ",".
Also support wildcard(\*),like ehcache\*.

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This PR originally started at #19, @daimingzhi and I have a long discussion about this topic there.

[wu-sheng]

To reviewers(@apache/skywalking-committers), when you merge this, please make sure we have @daimingzhi listed as a co-author of this PR. Because of squash and merge policy, this PR will belong to my account I am afraid. Let's keep the track correctly and respect our contributor.

[wu-sheng]

I noticed precious plugin test codes didn't verify this feature. Because Spring classloader includes target jars in the Spring fatjar, which means the httpclient jars are not loaded through ext-classloader.
Let me fix this test case.

[wu-sheng]

An update, -Djava.ext.dirs is obsolete. Since JDK9 you will see this

<JAVA_HOME>/lib/ext exists, extensions mechanism no longer supported; Use -classpath instead.
.Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

[wu-sheng]

@daimingzhi According to JDK 11 is widely used and recommended to use. JDK8 would be the only LTS JDK.

JDK8 EOF update

1. After January of 2019, only customers who have signed a support contract with Oracle will still receive security updates for Oracle JDK 8.
2. Maintenance for OpenJDK 8 is planned for at least another 4 years, until September 2023. 2 years left for EOL.

So, generally, according to the roadmap of JDK, java.ext.dirs should be replaced by cp or classpath. I will close this PR with original one as well. But I will add a FAQ doc to say what is going on, if user wants to try this draft feature, feel free to download and try.

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Same as this reason, I would NOT bump up agent version to v9. Let's stay in v8 for longer.