add the generate release announcement message command

[tashima42]

command to generate the rancher release announcement messages:

release generate rancher release-message --tag v2.8.8-alpha2 --previous-tag v2.8.8-alpha1 --prime-only --action-run-id 10857046301

`v2.8.8-alpha2` is available based on this commit ([link](https://github.com/rancher/rancher/commit/release/v2.8))!
* Link of commits between last 2 RCs. ([link](https://github.com/rancher/rancher/compare/v2.8.8-alpha1...v2.8.8-alpha2))
* Completed GHA build ([link](https://github.com/rancher/rancher/actions/runs/10857046301)).
* Images with -rc:
    * rancher/aks-operator v1.2.5-rc.1
    * rancher/cis-operator v1.0.15-rc.2
    * rancher/eks-operator v1.3.5-rc.1
    * rancher/gke-operator v1.2.5-rc.1
    * rancher/rancher-webhook v0.4.11-rc.2
    * rancher/security-scan v0.2.17-rc14
    * rancher/system-agent v0.3.9-rc.4-suc
    * rancher/wins v0.4.18-rc2
* Components with -rc:
    * SYSTEM_AGENT_VERSION v0.3.9-rc.4
    * WINS_AGENT_VERSION v0.4.18-rc2
    * AKS-OPERATOR v1.2.5-rc.1
    * EKS-OPERATOR v1.3.5-rc.1
    * GKE-OPERATOR v1.2.5-rc.1
    * RKE v1.5.13-rc.2