Add concrete description for EmbeddedEventLoop

[aryan-25]

Motivation:

We want to avoid going through EventLoopGroup's String(describing: self) when obtaining the description of an EventLoopGroup or EventLoop.

The concrete MultiThreadedEventLoopGroup and SelectableEventLoop already implement a description property. EmbeddedEventLoop does not.

Modifications:

Added a concrete description property in EmbeddedEventLoop.

Result:

The description of EmbeddedEventLoop can be obtained without going through String(describing: self).