Implement k0s etcd member-update command

[Danil-Grigorev]

Description

Having this command in place would allow to perform member peerURL update without using etcdctl when such operation is required.

It can be used as:

k0s etcd member-update --member-name <hostname> <peerURLs>

Fixes #6582

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[twz123]

I wonder if there's another way to achieve this. Maybe a higher level thing like "k0s rejoin" or sth. like this.