Closes#2294
Support AWS's conditional-write condition keys in both S3 bucket policies and IAM identity policies, so a policy can require that an upload or delete carry an `If-Match`/`If-None-Match` precondition.
`requestConditionContext` now populates both keys from the request headers, which covers both policy types at once: the same map is evaluated in-process for bucket policies and shipped to the IAM service for identity policies. Only the three requests whose preconditions the gateway actually enforces contribute - PutObject, CompleteMultipartUpload and DeleteObject. Copies, form uploads, DeleteObjects batches, upload parts and the sub-resource writes all ignore the headers, and a policy must never grant on a precondition that won't be checked; reads are excluded for the same reason, since GET and HEAD take these headers as ordinary HTTP cache preconditions. The value is the ETag with its surrounding quotes stripped, matching what the gateway enforces against.
Bucket policies validate condition keys against a fixed catalogue at `PutBucketPolicy` time, so both keys are added there with the action sets AWS accepts: `s3:if-match` on `s3:PutObject` and `s3:DeleteObject`, `s3:if-none-match` on `s3:PutObject` alone. Identity policies validate only the operator vocabulary, matching AWS, so they need no change.
niksis02
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