feat: scaffold libs/server-sdk-dynamodb-source

[beekld]

Summary

Scaffolds a new libs/server-sdk-dynamodb-source library that mirrors libs/server-sdk-redis-source, plus the aws-sdk-cpp dependency wiring.

No functional code. Proves that the AWS C++ SDK builds cleanly in our CMake/CI setup.

What's in the box

• cmake/aws-sdk-cpp.cmake — FetchContent for aws-sdk-cpp@1.11.805 with BUILD_ONLY=dynamodb, ENABLE_TESTING=OFF, and GIT_SUBMODULES_RECURSE TRUE (the SDK still uses submodules for aws-crt-cpp and the underlying AWS C runtime libs). Build pruned to aws-cpp-sdk-dynamodb + aws-cpp-sdk-core + the CRT stack — no other AWS service clients compile.
• libs/server-sdk-dynamodb-source/ — CMake / package / Doxygen / README skeleton paralleling server-sdk-redis-source. The lone dynamodb_source.cpp references Aws::DynamoDB::DynamoDBClient inside a never-called function so the link against aws-cpp-sdk-dynamodb is exercised by this PR rather than deferred.
• LD_BUILD_DYNAMODB_SUPPORT CMake option (default OFF) wired into the top-level CMakeLists.txt.
• scripts/build.sh — special-cases the new target to flip LD_BUILD_DYNAMODB_SUPPORT=ON, matching the existing redis/otel pattern that the .github/actions/ci composite already uses.
• .github/workflows/server-dynamodb.yml — build-only CI matrix on ubuntu-22.04 / macos-15 / windows-2022, so "AWS SDK builds cleanly in CI" is actually verified by this PR.
• release-please-config.json + .release-please-manifest.json — new package registered at 0.1.0 with bump-minor-pre-major: true (matching server-sdk-otel, the only other pre-stable package).
• .github/workflows/manual-sdk-release-artifacts.yml — new package added to the manual release options.

Out of scope (deferred to later PRs)

• DynamoDBDataSource implementation
• IBigSegmentStore interface + DynamoDB/Redis big-segment implementations
• Per-package release / SLSA-provenance jobs in .github/workflows/release-please.yml — deferred until the package has functional artifacts to release.

Test plan

• CI is green on Linux, macOS, and Windows. Linux is verified locally; Mac and Windows are first verified by CI on this PR
• Local build sanity-check passes: cmake -G Ninja -B build -DLD_BUILD_DYNAMODB_SUPPORT=ON . then cmake --build build --target launchdarkly-cpp-server-dynamodb-source produces liblaunchdarkly-cpp-server-dynamodb-source.a.
• Default-off check: a vanilla cmake -G Ninja -B build . produces no dynamodb or aws-sdk targets in ninja -t targets all.

Risks / things reviewers should look at

• The aws-sdk-cpp tag pin (1.11.805) is recent.
• AWS SDK adds noticeable cold-build time; ccache/sccache in CI will absorb subsequent runs. If this becomes a problem we can revisit pre-built artifacts later.
• scripts/build.sh now has three "special-case" branches (redis, otel, dynamodb).

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Note

Medium Risk
Introduces a new optional build path that fetches and links the AWS C++ SDK via CMake/FetchContent, which can impact build times and CI stability across platforms. Functional behavior is minimal, but release/CI plumbing changes could affect packaging if misconfigured.

Overview
Adds a new scaffold library, libs/server-sdk-dynamodb-source, with a placeholder API and CMake target (launchdarkly::server_dynamodb_source) that links against aws-sdk-cpp (DynamoDB-only) to validate dependency wiring.

Wires this component into the repo’s build and release tooling: new LD_BUILD_DYNAMODB_SUPPORT toggle in the top-level CMakeLists.txt, scripts/build.sh special-casing to enable it only for the DynamoDB targets, a dedicated CI workflow (server-dynamodb.yml) for Linux/macOS/Windows build-only checks, and updates to release-please + the manual artifact release workflow to recognize the new package at version 0.1.0.

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