Add package type to manifest file

[ruflin]

Currently the type is not used and the default type is 'integration'. But this gives us the option in the future to also introduce packages which are not integrations, for example for solutions.

[ruflin]

@jfsiii @skh This currently has no effect on the UI I assume but probably good to know it is there.

[jfsiii]

I see the docs/api/* files were updated. Out of curiosity, is that automatic when you build/merge/deploy or do you need to remember to "rebuild"?

This change isn't breaking but it is what I was referring to in elastic/kibana#38598 I really want to work out some coordination which removes the human component -- the need to ping one another.

[ruflin]

To update these files I need to run go test . -generate as otherwise my tests fail. +1 on having some automation here. A human thing that will also reduce pings will be that we start using the Changelog and do proper versioning. My plan is to start this today.