This commit adds three new commands for managing plugins. The `list`
command will show which plugins are currently registered and their
enabled state. The `enable` and `disable` commands can be used to enable
or disable plugins.

A disabled plugin will not show up to the PluginManager when it iterates
over available plugins of a particular type.

The purpose of these commands is to provide more visibility into
registered plugins and allow users to disable plugins for experimental
perf reasons.

There are a few limitations to the current implementation

1. Only SystemRuntime and InstrumentationRuntime plugins are currently
supported. We can easily extend the existing implementation to support
more types. The scope was limited to these plugins to keep the PR size
manageable.

2. Only "statically" know plugin types are supported (i.e. those managed
by the PluginManager and not from `plugin load`). It is possibly we
could support dynamic plugins as well, but I have not looked into it
yet.