Fixes#13
This builds on top of extensions catalog (see
https://github.com/siderolabs/extensions/pull/225), and existing support
for specifying extension in the flavor.
Image Service resolve the list of extensions requested for a specific
version of Talos into a list of container images, pulls them, and
attaches them to the image request.
Image Service also provides endpoints to get information about available
Talos versions, supported extensions for each version, etc.
I also refactored a bit flow around fetching & verifying image to re-use
it in other flows, added support for authentication to the registry.
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
