Support tar.gz archives

[thompson-tomo]

Describe the problem/challenge you have
I want to be unpack the source code of the github release which i am syncing.

Describe the solution you'd like
The supported archive type extended to include tar.gz similiar to what is proposed in #207

Anything else you would like to add:
I am aware that i could sync git repo's directly however the use case i am interested in as following.

Semantic conventions perform a github release, projects which consume the conventions would get a renovate pr to update the version. As part of this renovate pr ci tasks run to generate the new artifacts based on the latest semantic conventions.

My config is:

apiVersion: vendir.k14s.io/v1alpha1
kind: Config

directories:
- # path is relative to `vendir` CLI working directory
  path: semantic_conventions/tmp
  contents:
  - path: semconvrepo2
    githubRelease:
      # slug for repository (org/repo) (required)
      slug: open-telemetry/weaver
      # use release tag (optional)
      # optional if tagSelection is specified (available in v0.22.0+)
      tag: v0.19.0
      disableAutoChecksumValidation: true
      # specifies which archive to unpack for contents (optional)
      unpackArchive:
        # (required)
        path: source.tar.gz

Currently when i run the vendir sync command I get the following:

Fetching: semantic_conventions/tmp + semconvrepo2 (github release open-telemetry/weaver@v0.19.0)

vendir: Error: Syncing directory 'semantic_conventions/tmp':
  Syncing directory 'semconvrepo2' with github release contents:
    Expected known archive type (zip, tgz, tar)

Note this is an extension/complementary to #432

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