fix(cloudfront): serve S3 REST origins from the local process

[Sorttech]

An origin whose domain is a bucket REST endpoint
(<bucket>.s3.<region>.amazonaws.com -- what S3OriginConfig origins and
CDK's S3BucketOrigin carry) resolves in real DNS, so the data plane
proxied the fetch to real AWS S3 and the distribution never served the
bucket. Route it to this process like an S3-website origin, keeping the
bucket domain in Host so the S3 front door resolves it virtual-hosted
style. An explicit CustomOriginConfig still wins.

Test plan

Regression test: serves_static_from_s3_rest_origin in crates/fakecloud-e2e/tests/cloudfront_dataplane.rs

Verification

On this exact head, rebased onto current main:

• cargo fmt --all --check - clean.
• cargo clippy --workspace --all-targets -- -D warnings - clean.
• cargo test --workspace excluding fakecloud-e2e, fakecloud-conformance,
  fakecloud-tfacc and fakecloud-parity (the same set CI's test job runs),
  plus cargo test -p fakecloud-conformance --lib - 9931 passed, 0 failed.
• cargo test -p fakecloud-e2e --test cloudfront_dataplane - the regression
  test above passes against a freshly built target/debug/fakecloud.

Found by deploying a CDK CloudFront + S3 SPA against fakecloud.

[Sorttech]

Retargeted upstream as faiscadev#2559.

[Sorttech]

Retargeted upstream as faiscadev#2559.