Migrate all mirrored sources from Nexus to GitHub repositories

[dervoeti]

Update the “upload new version” process, which currently relies on shell scripts, to use GitHub instead. Ensure all historical versions are preserved if necessary.

[dervoeti]

I would probably start by forking all upstream repos into the stackabletech organization (some are already there). The question is how to sync upstream tags and branches in these forks.

I have three ideas:
a) Create a GH action that runs periodically and syncs all tags and branches.
b) Integrate that logic into patchable, probably to patchable init. patchable would either create the fork or just assume that it exists. Then it would pull/push the specified ref (tag, branch or commit-id) from upstream to the fork and set the fork as upstream repo. It would need push permissions in the forked repo though.
c) Document how to do the pull/push manually before running patchable init

@nightkr WDYT?

[dervoeti]

Results from a quick discussion with @nightkr:

• I'll create a draft implemenation for patchable init --sync-mirror. Forks need to be created manually.
• Do we want to put the forks in a separate GH org?
• Do we want to use gh forks or create our own mirror repos?

[dervoeti]

We changed some things regarding patchable in #1070:

• A default mirror repository can be specified for each product (patchable init product druid --upstream https://github.com/apache/druid.git --default-mirror https://github.com/stackabletech/druid.git)
• The source code of each product version can be mirrored in that repository (patchable init version druid 31.0.1 --base=druid-31.0.1 --mirror)
• patchable will go ahead and copy the branch or tag specified in --base to the mirror repository as part of the init version subcommand
• patchable checkout (used e.g. inside Dockerfiles) will clone from that mirror repository and apply the patches on top of it

When adding a new product that should be mirrored, the mirror repository needs to be created manually on Github (using the "fork" functionality).