Add support for extensions in pack Buildpack downloader

[itsdarshankumar]

Summary

Adds feature to add packaged extensions while creating a builder

Output

Before

Used to throw SIGSEV

Related

Resolves #1684

[natalieparellano]

Thank you @itsdarshankumar - this looks great! Would it be possible to add a test case to pkg/buildpack/downloader_test.go? Something along the lines of:

		when("package image lives in docker registry", func() {
			when("image key is provided", func()
				it("should succeed", func() {

The other tests in this context (for buildpacks) would probably be redundant for extensions.

[itsdarshankumar]

Thank you @itsdarshankumar - this looks great! Would it be possible to add a test case to pkg/buildpack/downloader_test.go? Something along the lines of:

		when("package image lives in docker registry", func() {
			when("image key is provided", func()
				it("should succeed", func() {

The other tests in this context (for buildpacks) would probably be redundant for extensions.

sure, I'll add the test cases

[natalieparellano]

Awesome work @itsdarshankumar! IMO Codecov is being too picky... this will really smooth out the experience of using extensions for end users!

[jjbustamante]

Awesome @itsdarshankumar !!!