✨ Allow to perform manual ETCD snapshots

[Danil-Grigorev]

What this PR does / why we need it:

This PR completes the initial setup of the controller with functionality to create manual snapshots on user demand, using the system-agent and RKE2 cli, following the docs.

This comes with some problems, like inability to specifically name the snapshot and deterministically connect it to the ETCDMachineSnapshot replica, created by the user, but the issue lays in lacking options in k3s/rke2 CLI. In the scenario when the snapshot is named “my-snapshot”, the actual name looks like: local-my-snapshot-rke2-control-plane-mnf8z-1731675906-31aa01 and may easily overlap with a snapshot named as just “snapshot”. (Edit: currently matches against snapshotName and machineName)

This also makes it hard to connect the creationTimestamp of the snapshot on the child cluster with the originally invoked ETCDMachineSnapshot resource execution, as no additional info can be stored on the snapshot instance when calling the CLI. (Edit: storing snapshot timestamp in the list now)

Final snapshot location and other infor is created and is available on the list within the automatic snapshot record (one per cluster).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #725

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[alexander-demicev]

thanks! looks good to me

[yiannistri]

LGTM