feat: add ActionType generic constraint for policy actions

[harshavardhana]

Description

Adds policy.ActionType, a generic constraint that is the union of the policy
action types:

type ActionType interface {
    Action | AdminAction | STSAction | KMSAction | TableAction | VectorsAction | MemoryAction
}

It lets generic auth helpers accept any typed action without forcing callers to
convert to Action at each call site, while a bare string stays excluded
(so it's tighter than ~string).

Motivation

Consumers (e.g. MinIO AIStor) have request-auth helpers typed on policy.Action.
As more service action types are added (TableAction, VectorsAction,
MemoryAction, …), callers must wrap each typed constant in policy.Action(...).
A shared constraint lets those helpers become generic over ActionType and drop
the per-call conversions, keeping the single Action conversion at the boundary
where the policy engine evaluates.

Changes

• policy/action-constraint.go — the ActionType union constraint.

No behavior change; type-level addition only. go build ./... and
go vet ./policy/... pass.

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Reviewing files that changed from the base of the PR and between 4575139 and 6f5e79b.

📒 Files selected for processing (1)

• policy/action-constraint.go

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