chore: Add async shim layer for concurrency and HTTP transport

[jsonbailey]

Summary

Adds the async shim layer that later async slices build on. These modules are dormant — nothing in main imports them yet — so this PR adds no runtime surface to the synchronous SDK beyond the new (unimported) modules and their tests.

This is part of the async Python SDK epic (SDK-60), shipped as a stacked series of focused PRs. SDK-2601.

What's added

• ldclient/impl/shims/aio.py — async concurrency primitives: AsyncEvent, AsyncLock, AsyncRWLock, AsyncQueue, AsyncRepeatingTask, AsyncCallbackScheduler, AsyncTaskRunner, AsyncWorkerPool, plus spawn_handle/join_handle, resolve, and iterate. Each wraps a piece of fiddly asyncio plumbing the async shells would otherwise inline; the sync side uses the equivalent stdlib/SDK primitives directly, so there is no sync twin.
• ldclient/impl/shims/aio_transport.py — async HTTP/SSE transport over an aiohttp ClientSession: make_client_session (SSL/cert/proxy setup from the SDK HTTP config), AsyncHTTPTransport (session-ownership-aware request/close), and AsyncSSEFactory (configured AsyncSSEClient for streaming).
• ldclient/impl/shims/transport_types.py — TransportResponse, the uniform response type shared by the transport shims.
• ldclient/testing/impl/test_shims.py — unit coverage for the concurrency primitives and the transport (SSL context application, session ownership, SSE client creation).

What will consume the shims later

• aio.py → async big segments, the flag tracker, the event processor, FDv1/FDv2 polling and streaming, and the data systems.
• aio_transport.py → async FDv1 streaming/polling, the event processor, and the FDv2 sources.

Packaging

Introduces the [async] optional-dependencies extra (aiohttp) and adds aiohttp + pytest-asyncio to the dev group, with asyncio_mode = "strict" so the new async tests run under pytest-asyncio in CI. No change to the eventsource pin, the redis pin, or the project version.

Testing

• uv run pytest ldclient/testing/impl/test_shims.py — 26 passed
• Full unit suite (database tests skipped) — 1041 passed, 194 skipped
• uv run mypy ldclient — success, no issues
• uv run isort --check ldclient / pycodestyle — clean