Add ecs.version field

[pgomulka]

it would be great if there was a public constant/method that would allow getting an ECS version that the lib is compatible with.

[felixbarny]

This would be the minimal ECS version right? Meaning the lowest version number that contains all the fields used by the lib.

Btw, this is already specified here but the Java lib didn't implement it yet: https://github.com/elastic/ecs-logging/tree/master/spec#minimum-viable-product

[pgomulka]

lowest version number that contains all the fields used by the lib.

Assuming newer versions are always bwc with older version then yes. What if a field is removed from ecs at some point?
the documentation is not precise on what guarantees should we expect when querying a newer version
https://www.elastic.co/guide/en/ecs/current/ecs-ecs.html

[felixbarny]

What if a field is removed from ecs at some point?

If a non-experimental field is removed, I'd consider that a breaking change that would be released in a new major version of ECS.

If a new ECS major version does not impact the subset of fields used by the logging lib it should also be compatible, too.