add ECS support

[kaisecheng]

This PR add ECS support to the filter.
It adds a warning if target is not set in ECS mode

Fixed #80

[kaisecheng]

I haven't added the table to compare the columns of ECS disabled and ECS v1 because the field name is depending on the value of target which has no default value

[kares]

I understand the intention but from a plugin user perspective the docs seems confusing.
Maybe we should just include the same message as the one that gets logged to explain?

The plugin behaves the same regardless of ECS compatibility, except issuing a warning when `target` isn't set.
It is recommended to set the `target` option to avoid potential schema conflicts.

(just a draft - probably need rewording if we decide to use it)

[kares]

for generic plugins such as this one it would be nice to wait for logstash-plugins/logstash-mixin-ecs_compatibility_support#6 (just in case we need to change the message or do more target checks)

while at it we might also want to add field reference validation for the target

[kares]

I understand the intention but from a plugin user perspective the docs seems confusing.
Maybe we should just include the same message as the one that gets logged to explain?

The plugin behaves the same regardless of ECS compatibility, except issuing a warning when `target` isn't set.
It is recommended to set the `target` option to avoid potential schema conflicts.

(just a draft - probably need rewording if we decide to use it)

[karenzone]

Comments in line for your consideration. I think it's important to add the "...regardless of ECS compatibility" wording to make the meaning more clear. You can decide if you like the tip formatting or not. :-)

[karenzone]

Suggested change

	The plugin behaves the same except giving a warning when ECS is enabled and `target` isn't set.
	It is recommended to set the `target` option to avoid potential schema conflicts.
	The plugin behaves the same regardless of ECS compatibility, except giving a warning when ECS is enabled and `target` isn't set.
	TIP: Set the `target` option to avoid potential schema conflicts.

[karenzone]

I like the way y'all handled this. You can use a TIP to put more emphasis on the impact of ECS. If you want to try this, it will render like this: