#154 Feat: name forEach items by a key

Description of your changes

Adds an optional key to ForEach, so a loop's steps can be named by each item's values rather than its position.

forEach:
  from: targets            # [{cluster: prod-us, namespace: shop}, ...]
  key: [cluster, namespace]

Looped	Without key	With key: [cluster, namespace]
a step deploy	deploy-0	deploy-prod-us-shop
a group rollout, sub-step verify	rollout-0-verify	rollout-prod-us-shop-verify

• key is a list of paths into each item, the same dotted syntax as inputs[].from. One field is still a list (key: [name]), so the CRD and the docs have one shape.
• The values are joined with -. The engine checks each part is a string or an integer, none is missing, the joined key uses step-name characters and is unique in the loop; the schema bounds the list at one to eight paths.
• The Workflow CRD is regenerated.

This is the API half. The engine is kubevela/workflow#273 and the Application support kubevela/kubevela#7468.

How has this code been tested

• A round-trip test for key, its omission when unset, and deepcopy independence.
• The engine and KubeVela changes are built and tested on top of this commit (unit, envtest and lint in both).

Special notes for your reviewer

Merge order: this PR first, then kubevela/workflow#273, then kubevela/kubevela#7468. The other two carry a temporary replace onto this branch until this merges.

---

Summary by cubic

Adds an optional key to ForEach so loop steps are named by each item's values instead of its position. With key: [cluster, namespace] over items like {cluster: prod-us, namespace: shop}, a step deploy becomes deploy-prod-us-shop instead of deploy-0; the Workflow CRD is regenerated.

• key takes one to eight dotted paths into each item; values must be strings or integers, be present, and be unique after joining with -.
• This is the API half only. The engine (kubevela/workflow) and Application (kubevela/kubevela) support land in follow-up PRs, so this PR must merge first.
• The field is optional and omitted when unset, so existing loops behave as before.