Added support for `http` fetch within `githubRelease`

[niallthomson]

This change allows the optional use of an http section within a githubRelease block, which can be used to directly create the URL of the asset to download with metadata from the GitHub release such as the tag name. The url parameter of http can interpolate the tag of the GitHub release using the {tag} token.

This is useful for several use-cases, primarily when a project publishes GitHub releases but does not include files as assets directly. It will also provide a solution for #145.

Example usage:

apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
- path: vendor
  contents:
  # templates http url with release tag
  - path: github.com/kubernetes/kubectl
    githubRelease:
      slug: kubernetes/kubernetes
      tagSelection:
        semver:
          constraints: "~v1.23.x"
      http:
        url: https://dl.k8s.io/release/{tag}/bin/linux/amd64/kubectl

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[niallthomson]

Looks like the tests are failing due to API request limits or am I missing something?

[cppforlife]

@niallthomson would you mind rebasing on develop. i pushed a fix for symlink failures we've seen (seems like github action error detecting logic somehow changed and starting seeing it as error).

it does seem like there is a rate limit problem if it doesnt "disappear" we can configure to use GITHUB_TOKEN.

[niallthomson]

@cppforlife rebased and pushed

[cppforlife]

test failure is unrelated. im going to merge and resolve the test failure separately by setting github token in the CI. thanks for contributing @niallthomson.

[vmwclabot]

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