Add support for log4j2 property substitution and lookup variables

Hi,

`EcsLayout` component is great for log4j2.
But for now, we can't inject dynamic values in `EcsLayout` attributes.
For instance; `serviceVersion` and `serviceNodeName`

So it would be great to add support for log4j2 [property substitution](https://logging.apache.org/log4j/2.x/manual/configuration.html#PropertySubstitution) and [lookup variables](https://logging.apache.org/log4j/2.x/manual/configuration.html#RuntimeLookup) in EcsLayout attributes!

It would allow to write this EcsLayout configuration for instance:
``` xml

```