Introduce packaging to Docker Mixin

[malcolmholmes]

A proposal for a packaging format for observability packages that is back-compatible with Jsonnet Mixins.

[malcolmholmes]

to test, render this:

• First, create a file, m.jsonnet that makes the hidden elements in a mixin visible:

local mixin = import 'mixin.libsonnet';
{
  grafanaDashboards: {
    [key]: mixin.grafanaDashboards[key]
    for key in std.objectFields(mixin.grafanaDashboards)
  }
}

• Next, execute: jsonnet -J vendor m.jsonnet
• This will give you the json output from the mixin.
• Alternatively, execute: jsonnet -J vendor polly.libsonnet to get the package contents.

[v-zhuravlev]

I think queries are better packed as targets (query + legend)

[nikimanoledaki]

Closing older PRs in this repo that seem unmaintained, sorry folks