tar: read directly from stdin

[kvaps]

Description

When using --context=tar://stdin option kaniko first writing tar.gz file on the drive, then unpacking it.
This PR changes this behavior to allow unpacking tar archive by reading it directly from stdin.

This also enables opportunity to reuse stdin to run debug shell #1727

Submitter Checklist

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review them:

• Includes unit tests
• Adds integration tests if needed.

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- reading context from 'tar://stdin' improved by unpacking directly from the piped stream

[tejal29]

please rebase to run the tests!

[kvaps]

PR rebased to v1.7.0, new docker images with this change prepared:

ghcr.io/kvaps/kaniko-executor:v1.7.0
ghcr.io/kvaps/kaniko-executor:v1.7.0-debug
ghcr.io/kvaps/kaniko-warmer:v1.7.0

source code of fork in master branch:
https://github.com/kvaps/kaniko/

[imjasonh]

This change seems like something that should be accompanied by an integration test, wdyt?