flavors, metdata/image/oci download links

[yeoldegrove]

What this PR does / why we need it:
gardenlinux/gardenlinux#2523 and gardenlinux/gardenlinux#2427 mention the need and ideas to to put the following in the GLRD:

• image manifest download links (on s3 bucket)
• image download links (on s3 bucket)
• OCI image download links (in oci/image registry)

This is all heavily "flavor" dependent information, so the flavors that are built and published for a non-virtual certain release (patch, nightly, dev) need to be tied to the GLRD entry.
The additional information that is needed to meet the 3 new features is heavily redundant ...

• prefix of links to s3 bucket
• prefix of oci image

... or can be derived from the type of the image (vm image or container image) and is already inside the flavor metadata:

• links to s3 vs. links to oci

This is why in this approach the only new entry to the schema of glrd-mange and the produced json files is the addition of flavors as a dict of strings.
All other virtual fields inside flavors for metadata and image and also the oci field are generated by glrd on the fly.

Which issue(s) this PR fixes:
Fixes #2523 and #2427

Special notes for your reviewer:

• This PR also introduces poetry for project and dependency management to better track dependencies to other python packages in git repo's.

• Needs merge and update of dependencies in add parsing of flavors python-gardenlinux-lib#87.

• After the merge of this PR, the existing GLRD data needs to be updated for these fields:

• • flavors
  • oci

• This is where the new glrd-update tool can help.

glrd-update --s3-download --s3-update

[yeoldegrove]

@NotTheEvilOne I opened #4 to coordinate the refactoring.
Minor things and the exception handling are pushed. A few questions remain.

[NotTheEvilOne]

Looks better now. Thank you :)