Rewrite bash scripts into go

[dergeberl]

Open Tasks:

• [x] add docs for local setup and testing
• [x] check README.md
• [x] reduce log from pathcontroller
• [x] remove shell scripts 🎉
• [x] check memory usage - improved for VPNShoot
• [x] check container images (before the PR there was less other software, like ls)
  • [x] check if it is possible to have a dev image with useful tools

What this PR does / why we need it:

This PR rewrites the bash scripts into go. This will made it easier to adopt to new features in the future.

• Some ENVs are different or removed, this needs to be rolledout together with the changes in the gardener/gardener https://github.com/gardener/gardener/pull/9774
• The initContainers use a subcommand
• Everything that was done with iproute2 is now done with the netlink package
• Merge Dockerfiles into one Dockerfile

Co-authored-by: @MartinWeindel Co-authored-by: @hown3d Co-authored-by: @mreiger

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer: ⚠️ The pipeline jobs for building images are failing as changes in .ci/pipeline_definitions are not yet used. They become only active after merging to master branch.

⚠️ This is a breaking change and needs also changes to gardener/gardener

Release note:

Rewrite bash scripts into go. 
Log output changed. Expected ENVs from the containers changed. More memory is needed because of the change to go.

[gardener-robot]

@dergeberl Thank you for your contribution.

[gardener-robot-ci-3]

Thank you @dergeberl for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

[MartinWeindel]

Regarding memory usage these are my major findings:

• Investigating the heap usage of the Go executables found only very small heap size about 1-2 MB. So this not the cause.
• The RSS (Resident Set Size) was still large if the openvpn was run in a separate container. There seems to be an memory overhead caused by Rosetta on Macbooks with M1/M2 CPUs when using local Gardener setup
• Memory consumption drops significantly if local Gardener setup run directly on Linux machine
• Remaining memory consumption seems to be related to pages of the go executable itself
• After moving the start/restart loop back into a minimal shell script (commit https://github.com/gardener/vpn2/pull/84/commits/09bf6f6f7810edacd32773586cffbd654c0edf6f), memory usage was back to same level as before the rewrite.

vpn-seed-server-665cf779b9-52vwb             3m           19Mi
---
vpn-shoot-f956748d-gv5h5                     5m           1Mi

Without the script, on Linux the memory consumption was ~ 6Mi

[gardener-robot]

@dergeberl You need rebase this pull request with latest master branch. Please check.

[axel7born]

With ha vpn we have the shoot-client container running in the seed in the kube-apiserver pod. This is already confusing. The path-controller is only needed in seed. Maybe, we should we use the opportunity to separate seed-client and shoot client? Or change the name from shoot-client to vpn-client?

[MartinWeindel]

With ha vpn we have the shoot-client container running in the seed in the kube-apiserver pod. This is already confusing. The path-controller is only needed in seed. Maybe, we should we use the opportunity to separate seed-client and shoot client? Or change the name from shoot-client to vpn-client?

As we need to rename the images anyway to be able to switch back to the old implementation, I propose to use vpn-client instead of shoot-client for package and image name and similar vpn-server instead of seed-server.

I don't see a big win in separating "seed-client" and "shoot-client", as they share many things.

[axel7born]

/lgtm

[MartinWeindel]

/lgtm