Automate format version update in package archetype

[jsoriano]

elastic-package create package creates a new package based on an embedded archetype. This archetype has a hard-coded format_version that we have to remember to manually update now.

Create some automation so this format version gets updated as new versions of the package spec are released.

Some options are discussed in #1152 (comment).

[ruflin]

I was thinking today about the new ecs build feature to pull in the dynamic templates. I would like see this becoming part of the build package eventually but the question I asked myself was: What ECS version will it use? Will we eventually have the same problem as with this update here?

Taking this to the next step: Lets assume we updated it in elastic-package. How will all the existing packages benefit from it? With go-modules, the latest versions can be discovered. Imagine a dev could run elastic-package update (just made up) and it would update all the dependencies like ecs version, format_version and other things we have. There will be cases, where it will end up in an invalid package and linting will complain, but it would provide an easy way for package devs to keep their packages up-to-date.

I know the above does not 100% in the scope of this issue, but I thought I share it here as I started to think about this more holistically.

[jsoriano]

Proposal about elastic-package update moved to its own issue: #1427