Support fetching artifacts by OS & architecture

[benmoss]

Describe the problem/challenge you have
I want my project to run on different platforms and so depending on which platform I am building for I want to download different artifacts.

For example, this vendir.yml pulls in kubectl for linux/amd64.

Describe the solution you'd like
I would prefer a solution where I don't have to download artifacts for all platforms, so maybe this needs to be an argument that's passed to vendir sync.

For the example given above it seems like we could allow this to work:

url: https://dl.k8s.io/release/{tag}/bin/{os}/{arch}/kubectl

Anything else you would like to add:
This probably only makes sense for certain kinds of artifacts: GitHub releases, HTTP maybe.

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[cppforlife]

may be also would be good to scope {os}/{arch} to something like {vendir.os}/{vendir.arch}, since in the example of githubRelease+http, tag is derived from that "local" context of githubRelease.

probably also makes sense to have binary{vendir.os_ext} so that on windows it's .exe vs on linux nothing.

[cppforlife]

interesting questions on this topic:

• vendir.lock.yml relationship to what os/arch is set
• how are values os/arch are set: auto, manually
• should vendir have rename functionality (e.g. ytt-darwin-amd64 => ytt)

[mamachanko]

+1 to this