Setup init improvements

[teodora-aleksovska]

This PR includes improvements to enabler setup init command. It now checks whether binaries already exist before downloading them.

After running enabler setup init, i received this output:

teodora@teodora-ThinkPad-T16-Gen-1:~/Project/enabler$ enabler setup init
Bin folder already exists. Continue...
-kubectl already exists at: bin/kubectl
helm already exists at: bin/helm
istioctl already exists at: bin/istioctl
kind already exists at: bin/kind
skaffold already exists at: bin/skaffold

[gocemitevski]

@teodora-aleksovska please describe what happens is some of the binaries does not exist - the mixed case scenario.

[teodora-aleksovska]

@gocemitevski after running the enabler setup init command, it will check if the binaries exist in the bin folder. If some of them do not exist, this command will download them.

Before these improvements, the downloading process worked correctly every time after running enabler setup init.

[gocemitevski]

@teodora-aleksovska OK. Please add all relevant descriptions in the README as well for each command. https://github.com/keitaroinc/enabler?tab=readme-ov-file#commands.

[gocemitevski]

The changes in the README should also be included in the feature-setup-init-improvements branch.

[teodora-aleksovska]

@gocemitevski additionally, this pull request includes the required changes in the README file and unit tests.