feat: add IBigSegmentStore interface + Redis and DynamoDB stores

[beekld]

Ticket: SDK-2363 · Follows #534

Summary

Adds the public IBigSegmentStore interface, the Membership / StoreMetadata value types, and concrete DynamoDB + Redis implementations. Schema strings match what the Relay Proxy writes.

auto redis_store = RedisBigSegmentStore::Create("redis://localhost:6379", "prefix");
auto dynamo_store = DynamoDBBigSegmentStore::Create("my-table", "prefix", options);

Design notes

• synchronizedOn parsing. Stored as DynamoDB N / Redis string. Both stores reject malformed values (non-numeric strings, wrong DynamoDB attribute type) rather than silently returning 0, matching the existing dynamodb_source.cpp row validation.
• No hashing here. The interface contract says callers pass the already-hashed base64 SHA-256 context key; the SDK will hash in the wrapper, not the store implementations.

Not in scope

• BigSegmentsBuilder config plumbing.
• BigSegmentStoreWrapper (LRU cache + staleness polling) and the hashing path.
• Evaluator wiring / replacing the rules.cpp big-segments TODO.

Test plan

• 7 Membership unit tests pass.
• 9 RedisBigSegmentStore integration tests pass against Redis 7 (docker).
• 8 DynamoDBBigSegmentStore integration tests pass against DynamoDB Local.
• Full server-sdk test suite (468 tests) still green.
• Schema constants match what Relay writes (verified against the LocalStack/Redis fixture data).

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Note

Medium Risk
New external-store read path for segment targeting with strict schema validation; not yet wired into evaluation, so production flag behavior is unchanged until follow-up PRs land.

Overview
Introduces a Big Segments persistent store surface for the server SDK: public IBigSegmentStore, inline Membership / StoreMetadata, and read-only Redis and DynamoDB backends that follow Relay’s key/schema layout (prefix scoping, include/exclude refs, sync metadata).

Stores perform point lookups by opaque context hash, build membership via Membership::FromSegmentRefs (inclusion wins on overlap), and return errors on malformed DynamoDB attribute types or non-numeric sync timestamps instead of silent empty/zero results. Config, caching wrapper, hashing, and flag evaluation wiring are not included in this change.

Integration tests cover membership/metadata behavior, prefix isolation, and error paths against Redis and DynamoDB Local.

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