Reduce the size of Ignition's binary (was: Vendor / rewrite support for protected assets with Azure Storage to not use the Azure SDK)

Bug

The size of the Ignition binary significantly increased with the 2.21.1 release, likely due to addition of the Azure SDK as vendored code: https://github.com/coreos/ignition/pull/1923

For Flatcar:

$ du --si ./bin/amd64/ignition-v2.20.0
38M	./bin/amd64/ignition-v2.20.0
$ du --si ./bin/amd64/ignition-v2.21.0
63M	./bin/amd64/ignition-v2.21.0

For Fedora CoreOS:

• https://koji.fedoraproject.org/koji/rpminfo?rpmID=42231971 (40.83 MB)
• https://koji.fedoraproject.org/koji/rpminfo?rpmID=41618061 (25.42 MB)

We could look at reducing the amount of vendored code, copying only the relevant part for support of this feature or re-implementing only what's needed for it.

Operating System Version

Flatcar & FCOS

Ignition Version

Ignition 2.21.0

Environment

What hardware/cloud provider/hypervisor is being used to run Ignition? Azure but this issue is for all platforms.

Expected Behavior

Ignition binary does not grow too much in size.

Actual Behavior

Significant increase in size.

Reproduction Steps

N/A

Other Information

See: https://github.com/coreos/ignition/issues/2011