Add build information of the package at build time

[marc-gr]

Would be useful to add build information of packages during build time (to show as part of the metadata shown in package registry).

Some use cases would be really simplified if we had this:

• Doing backports to older versions now that package-storage is deprecated
• Running benchmark comparisons against specific target versions (Report benchmark command can compare against a specific version elastic-package#1040)
• Know the version of elastic-package used to build the package, for example to know the specific ECS dynamic template imported (Add common dynamic mappings and properties from ECS automatically elastic-package#1073).
• Know when a package was built.
• From support perspective, more easily know the code a user is running.

[jsoriano]

This can be also useful in the context of elastic/elastic-package#1073, to know what dynamic templates were added to the package. Added use case to the description.