Add testo.Plugins function to obtain installed plugins

[metafates]

This PR adds a new testo.Plugins function:

// Plugins returns information about installed plugins
// in the given t instance.
//
// Returned slice follows plugins declaration order.
//
// For example:
//
//	type T struct{
//		*testo.T
//		*PluginBar
//		*PluginFoo
//		*PluginAllure
//	}
//
//	func (*Suite) Test(t T) {
//		testo.Plugins(t)
//	}
//
// Would return:
//
//	[]testoplugin.Plugin{*PluginBar(...), *PluginFoo(...), *PluginAllure(...)}
//
// NOTE: this function returns pointers to an actual plugin instances, so calling
// their methods is equivalent to calling T's methods.
func Plugins(t CommonT) []testoplugin.Plugin

Closes #32