Add ability to use ENVIRON for passing bootstrap token

[s3rj1k]

Adds two new flags to improve k0s installation and token management for both worker and controller:

• --start: Automatically starts the k0s service immediately after installation
• --token-env: Allows specifying the join token via an environment variable name instead of passing it via file

Example Usage

Install and start controller with token from environment

export K0S_TOKEN="your-token-here"
k0s install controller --token-env K0S_TOKEN --start

Install and start worker with token from environment

k0s install worker --token-env K0S_TOKEN --start

[github-actions]

This pull request has merge conflicts that need to be resolved.

[twz123]

Can we split this up into two PRs, one for the start flag, and the other one for the environment var? /cc #5253

[s3rj1k]

Can we split this up into two PRs, one for the start flag, and the other one for the environment var? /cc #5253

Probably, but I need both to be in binary at same time

[jnummelin]

Let's have these two things as separate PRs, and issues for them, so we can properly handle those as separate as they really are separate

[s3rj1k]

Let's have these two things as separate PRs, and issues for them, so we can properly handle those as separate as they really are separate

RN I still need this PR to test things, when I will finish testing related work against this, I'll prepare 2 separate PRs and move this one into Draft

[s3rj1k]

I've extracted --start feature into separate PR #6753

and --token-env into #6755