[Plugin/SDK] Add DeployTargetStatus and extend ExecuteStageResponse

[mohammedfirdouss]

What this PR does:
Adds a DeployTargetStatus type and extends ExecuteStageResponse with a DeployTargetStatuses []DeployTargetStatus field in pkg/plugin/sdk/deployment.go.

type DeployTargetStatus struct {
    Name    string
    Status  StageStatus
    Message string
}

type ExecuteStageResponse struct {
    Status               StageStatus          // unchanged — authoritative result
    DeployTargetStatuses []DeployTargetStatus // new, nil = no per-target detail
}

Why we need it:
Plugins that deploy to multiple clusters in parallel (e.g. kubernetes_multicluster) need to report per-target outcomes alongside the overall stage status. When a stage deploys to N clusters and one fails, operators currently see only "stage failed" with no indication of which cluster. The new field surfaces the per-cluster result.

This change to pkg/plugin/sdk/ will be picked up by the sync bot and propagated to pipe-cd/piped-plugin-sdk-go, after which a new SDK release tag can be cut and PR #6812 can drop its sdk_local/ shim.

Which issue(s) this PR fixes:
Related to #6446. Companion plugin PR: #6812.

Does this PR introduce a user-facing change?:

• How are users affected: Plugin authors gain an optional field to surface per-target status. Existing plugins require no changes, a nil slice is treated as "no per-target detail reported".
• Is this breaking change: No, the existing Status field is unchanged.
• How to migrate: N/A

[github-actions]

Thank you for contributing to PipeCD! The changes in this pull request will be part of the upcoming release!