First step towards enabling external plugins

[felixbarny]

What does this PR do?

Implements #937 (comment)

Makes it possible to load external plugins in an isolated class loader. The plugins depend on the apm-agent-plugin-sdk and the apm-agent-api artifacts to create a jar that can be place it into the plugins_dir. An example for such a plugin is in integration-tests/external-plugin-test.

As some parts are still missing, this is not mentioned in the changelog yet and the corresponding configuration is marked internal (thus not included in the generated docs).

• Creates apm-agent-plugin-sdk: used both by internal and external/community plugins
• I decided against fractoring out a apm-agent-tracer-impl module from apm-agent-core for now. It was more tricky than expected and not needed for external plugins.

Missing bits before we can open this to the public:

• Remove ElasticApmPlugin#indyPlugin, making indy plugins the only option
• stop shading Byte Buddy so that users can depend on elastic-apm-agent in their tests to activate apm-agent-api
  • agent has to be loaded in its dedicated class loader first to avoid conflicts with the application
• publish test module for apm-agent-core

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #1271 updated]

• Start Time: 2020-07-17T09:21:01.046+0000

• Duration: 42 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1461
Skipped	11
Total	1472

[felixbarny]

In a follow-up, we may want to include some of the CustomElementMatchers in the plugin SDK such as classLoaderCanLoadClass.

[felixbarny]

@elastic/apm-agent-java any changes you want to do in the ElasticApmInstrumentation class? After we publish the SDK, we can only add but never remove or rename methods.

One thing we might want to consider, although being a very minor thing, is to remove the get prefixes, for example from getTypeMatcher. It feels a bit awkward when writing an instrumentation because a type matcher is created, not retrieved when the method is called.

I have also changed the method name getImplementationVersionPostFilter to getProtectionDomainPostFilter to give it a more generic name.

[SylvainJuge]

First pass on the review, mostly minor comments and questions.