Add a "generate" command for producing integration data

[aspacca]

In the scope of the discussion at #984, we'd like to add a generate command to elastic-package

The command to run will be something like:

elastic-package data-generate --package=bar --dataset=foo

Implementation details:

• The generate command will consume the elastic-integration-corpus-generator-tool package
• For the first iteration the required artifacts (field yaml definitions, corpus-generator-tool configs and templates) will be fetched from the elastic-integration-corpus-generator-tool repo assets folder. Future iterations will define the proper entries in package-spec and the artifacts (or their equivalent) will be available directly from the package definitions
• For the first iteration the output will be a file in the current directly. Future iterations will add support for an --output parameter

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[aspacca]

Closed by #1122