Support custom trace ignore

Here is an optional plugin apm-trace-ignore-plugin

Introduce

• The purpose of this plugin is to filter endpoint which are expected to be ignored by the tracing system.
• You can setup multiple URL path patterns, The endpoints match these patterns wouldn't be traced.
• The current matching rules follow Ant Path match style , like /path/*, /path/**, /path/?.
• Copy apm-trace-ignore-plugin-x.jar to agent/plugins, restarting the agent can effect the plugin.

How to configure

There are two ways to configure ignore patterns. Settings through system env has higher priority.

1. Set through the system environment variable,you need to add skywalking.trace.ignore_path to the system variables, the value is the path that you need to ignore, multiple paths should be separated by ,
2. Copy/agent/optional-plugins/apm-trace-ignore-plugin/apm-trace-ignore-plugin.config to /agent/config/ dir, and add rules to filter traces

trace.ignore_path=/your/path/1/**,/your/path/2/**