Add option to unpack archive downloaded when using "http".

**Describe the problem/challenge you have**

When downloading archive file using `http` I need the archive file to be automatically unpacked.

**Describe the solution you'd like**

The `githubRelease` option has `unpackArchive` for something similar although in that case you need to designate the file to unpack. In the case of `http`, there is only the one asset file so could get away with a single boolean flag to indicate whether download should be unpacked.

```
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
- contents:
path: src/wrapt
- http:
url: https://github.com/GrahamDumpleton/wrapt/archive/refs/heads/develop.tar.gz
unpackArchive: true
path: .
```

**Anything else you would like to add:**

I am assuming that once unpacked you can still use `newRootPath` so as to indicate you want the final result to only include files which existed in a subdirectory of the archive.

For example, in the archive above the top level directory it unpacks into is `wrapt-develop`. I am hoping one can do:

```
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
- contents:
path: src/wrapt
- http:
url: https://github.com/GrahamDumpleton/wrapt/archive/refs/heads/develop.tar.gz
unpackArchive: true
path: .
newRootPath: wrapt-develop
```

so that top level `wrapt-develop` directory in the archive is eliminated and you get just the files in it.

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