aws_sdk_vendor: flattened source, no network fetch

[Minipada]

Parent

#421

What to build

Replace aws_sdk_vendor's ExternalProject_Add live git clone of aws-sdk-cpp/aws-crt-cpp with a flattened, plain-committed source tree (pinned 1.11.600) — not a git submodule (bloom's release snapshot does not reliably preserve submodule content). Build switches from a network-fetching ExternalProject to a local in-tree source build. Measure and report build time under --network none.

Acceptance criteria

• aws_sdk_vendor/CMakeLists.txt no longer performs any network fetch (no git clone)
• Vendored source is flattened plain files, not a git submodule
• Network-isolated build verification harness for vendor packages #423's network-isolated build check passes for aws_sdk_vendor
• Build time under --network none measured and reported in the PR description (compare against the ~20-minute network-included baseline documented in tools/e2e/Containerfile/tools/e2e: aws-sdk-cpp clone pulls unneeded submodules + full workspace layer invalidates on every source change #271 — informational, no hard pass/fail threshold since the buildfarm's actual job timeout isn't publicly discoverable)
• New ADR documents the flattened-source-vendoring decision, including the measured build time

Blocked by

• Network-isolated build verification harness for vendor packages #423

[Minipada]

Superseded by #434, implemented in #437. The no-network-access premise this issue was written on is false; aws_sdk_vendor fetches aws-sdk-cpp live via ament_vendor() instead. See docs/adr/0012-aws-sdk-vendor-flattened-source.md. PR #433 closed unmerged.