Add ability to reference waypoint.hcl variables between eachother

[evanphx]

This change adds the ability refer to other waypoint.hcl defined variables using HCL templating. Additionally, it adds a new class of variable called internal variables. These variables are not exposed to the application and are designed to be composed into other variables.

This functionally fully supports completing dynamic variables into non-dynamic ones, allowing for a better where internal dynamic variables are composed into a env variables:

config {
  internal = {
    dbhost = configdynamic("terraform-cloud", {
       organization = "foocorp",
       workspace = "production",
       output = "dbhost"
    })
  }

  env = {
    DATABASE_URL = "postgresql://${config.internal.dbhost}"
  }
}

[mitchellh]

This looks really good! Relatively simple overall, at least compared to what I thought would be required.

I think the one big question I have is how escaping works in this scenario (i.e. I literally want ${foo} in my value). This might just be handled by internal/pkg/partial. If so, I don't see any tests for it and I think we should definitely cover escaping tests at most levels.

[mitchellh]

Nitpick, we may want to turn this one into a bold heading style (see 0.3 or 0.2 or w/e changelog) but we can do that human-powered when we do a major release?

[briancain]

You should be able to use markdown here and go-changelog will respect it, so adding the * should be fine to do here if we want to bold it

[mitchellh]

Cool, yeah worth considering, I think this is a big feature.