[Fleet] Integrate package bundling process into kbn bootstrap

[kpollich]

In #122297 we introduced a step to Kibana's build process to bundle a configured set of packages and store their archives on disk. This process should also be exposed as part of Kibana's bootstrap process for development environments.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[joshdover]

@elastic/kibana-operations Any guidance on how we should do this? We currently have this only implemented for builds but would instead like to have this happen during yarn kbn bootstrap and then the build would be adjusted to only copy the files that were downloaded during bootstrap to the build directory.

Where is the best place to add a kbn:build npm package script? Can we do this in our own plugin's x-pack/plugins/fleet/package.json?

[spalger]

It feels like it makes sense for this to be the "build" process for a package... @mistic I feel like I've asked this before, but could we have a package which is mostly just a BUILD.bazel file that basically just downloads artifacts? Feels like something bazel must support,

[mistic]

@spalger I need to think about this better but I think we can declare an http_file rule from within the WORKSPACE and then reference this as a dependency in any other BUILD.bazel file as well as proceed with any extra action we need (like for example make it available somewhere in the bazel output folder)

[spalger]

I feel like we should be able to do this from a specific package, not just at the WORKSPACE level. Maybe we just need a custom rule for this, I have a couple other uses in mind.

[joshdover]

@spalger Is a custom rule something we (Fleet team) could help explore in the near term, or is this something you have time to look at?

[spalger]

If y'all want to do some research and try to implement it, I think it's great for more people to have experience here but I also expect that we could pull something together pretty quickly if @mistic thinks it's a worthwhile thing to have in the first place.