Provide options to install and configure system-agent

[Danil-Grigorev]

Motivation

Turtles does not have a CRD to facilitate user-defined configuration for the system-agent. In contrast, RKE2ConfigTemplate serves as a central resource for the cluster machines configuration, making it suitable to host additional configuration options for the system-agent. CAPRKE2 already has some logic in place to prevent machine re-rollout due to changes in the RKE2Config related to the system-agent installation.

Context: rancher/turtles#986 (comment)
Slack thread: https://suse.slack.com/archives/C047LDA35BM/p1736345542003029

Proposed solution

RKE2ConfigTemplate resource can provide options to enable and configure installation of the system-agent binary. This should simplify the process, as the RKE2Config is generated by the provider code from a user supplied RKE2ConfigTemplate, which additionally removes the need to host this logic in the webhook implementation.

cc @alexander-demicev @anmazzotti