feat: Add C bindings for DynamoDB LazyLoad source

[beekld]

Summary

Introduces DynamoDB C bindings. Before this PR, none of the DynamoDB integration was reachable from C -- neither LazyLoad source nor Big Segments store. This PR covers the LazyLoad source and the shared DynamoDBClientOptions surface; the Big Segments store follow-on will reuse the options builder introduced here.

New public surface:

• LDServerDynamoDBClientOptionsBuilder opaque handle for AWS DynamoDB client configuration.
• LDServerDynamoDBClientOptionsBuilder_New / _Free, and setters for _Region, _Endpoint, _AccessKeyId, _SecretAccessKey, _SessionToken.
• LDServerLazyLoadDynamoDBSource opaque handle.
• struct LDServerLazyLoadDynamoDBResult with source and error_message fields, plus LDSERVER_LAZYLOAD_DYNAMODBSOURCE_ERROR_MESSAGE_SIZE.
• LDServerLazyLoadDynamoDBSource_New(table_name, prefix, options, out_result) factory and LDServerLazyLoadDynamoDBSource_Free(source).

Tests:

• Unit tests for the options builder (create, set every field, free).
• Unit tests for successful source creation with and without the options builder (NULL means "AWS SDK default provider chain").
• End-to-end test against DynamoDB Local: creates a table, writes flags through the existing PrefixedDynamoDBClient fixture, wires the source through the C API, and verifies LDServerSDK_AllFlagsState reads the flags back. Mirrors the Redis LazyLoad e2e.

Design decisions worth flagging: the source type name uses config-section-first word order (LazyLoad + DynamoDB + Source) to match the existing Redis LazyLoad binding LDServerLazyLoadRedisSource. The client options type is named LDServerDynamoDBClientOptionsBuilder (with Builder suffix) even though the C++ side is a plain struct rather than a builder -- the C surface is builder-like from the caller's perspective (_New + setters + consumed by a factory), which matches the sibling LDServerHttpPropertiesTlsBuilder precedent in the same shape.

Stacked on #574 (Redis Big Segments store). Please review that PR first.

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Note

Medium Risk
New public C surface and AWS client credential configuration; behavior is read-only against Relay-populated tables but misconfiguration could affect flag loading in production.

Overview
Exposes the DynamoDB Lazy Load data source to C callers, matching the existing Redis LazyLoad binding pattern (LDServerLazyLoadRedisSource).

New C API: LDServerDynamoDBClientOptionsBuilder (region, endpoint, static/session credentials; unset fields use the AWS SDK default chain) and LDServerLazyLoadDynamoDBSource via LDServerLazyLoadDynamoDBSource_New(table, prefix, options, &result) with stack-allocated LDServerLazyLoadDynamoDBResult for success or a fixed-size error_message. Factories consume the options builder; callers free the source only if they do not hand it to LDServerLazyLoadBuilder_SourcePtr.

Implementation: Thin C shims in bindings/dynamodb/ delegate to DynamoDBDataSource::Create and DynamoDBClientOptions; CMake wires the new translation units into server-sdk-dynamodb-source.

Tests: Options builder lifecycle, source creation with options and with NULL options, plus a DynamoDB Local e2e that configures LazyLoad through the C API and asserts LDServerSDK_AllFlagsState reads seeded flags.

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