[Agent] Optional plugin mechanism

[ascrutae]

Currently SkyWalking loads all plugin jars from plugins folder to enhance class. However some plugins would probably affect the performance because of enhancing class. Or some of them are in beta. If the above plugins are put under the plugins folder, it will cause the user experience not so good and it's also not what the user wants to see. So I expect to create a new folder to put these plugins. And the user could copy the plugins jars to the plugins folder directly if they want to trace them.

[wu-sheng]

@ascrutae Since I just found you already did this in Spring bean plugins. Please provide a document about this.

[wu-sheng]

Done.