feat: add native AsyncDynavec client

[Isha-Zaka]

Description

Adds a native AsyncDynavec client backed by aioboto3 for high-concurrency async workloads while keeping the existing synchronous Dynavec API unchanged.

Related issue

Fixes #50

Changes

• Added async S3 Vectors and DynamoDB stores with async context-manager lifecycle and aclose().
• Added aupsert(), asearch(), and asearch_stream() using aembed_documents() / aembed_query(), including concurrent S3 and DynamoDB writes.
• Added the optional async extra, public AsyncDynavec export, async retry/rate-limit support, shared sync/async write preparation, and async/Moto integration coverage.
• Aligned the async paths with recent development changes for TTL and request rate limiting so sync and async behavior stay consistent.

Testing

• Full test suite passes locally: 817 passed, 2 skipped
• Ruff checks pass
• mypy passes with no issues in 62 source files
• Documentation updated, if applicable

Notes

Assume-role credentials currently reuse the existing synchronous STS resolution once during client construction, then pass the temporary credentials into the async session. AWS I/O performed by AsyncDynavec uses the async stores.

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[Kaap10]

LGTM! @Isha-Zaka.

Key highlights:

• Clean, non-blocking AsyncDynavec client with proper async context manager lifecycle (async with / aclose()).
• Robust aioboto3 backends for both DynamoDB and S3 Vectors with zero code duplication on shared logic.
• Full pytest-asyncio coverage (39 tests passed) and 100% clean typecheck and CI runs.

Merging this now.