Add optional async dependency and packaging support

[Mattsface]

Parent: #297
Depends on: #298

Goal

Add packaging support for async functionality without changing the dependency footprint for existing synchronous users.

Scope

• Select the async HTTP client based on the contract defined in Define the v1.1 async API and transport contract #298
• Add it as an optional dependency
• Expose an async installation extra, preferably:

pip install "python-mlb-statsapi[async]"

• Preserve normal synchronous installation:

pip install python-mlb-statsapi

• Define predictable behavior when async functionality is imported without the optional dependency installed
• Ensure wheel and sdist metadata include the extra correctly

Contract requirements from #298

The selected async HTTP dependency must be capable of supporting the public transport contract without leaking its own API into the stable surface:

• Async requests and cleanup must be awaitable
• One library-owned client must support multiple concurrent in-flight requests on the same event loop
• Caller-requested cancellation must be able to propagate as asyncio.CancelledError
• Request-level timeout configuration must be possible without mutating caller-injected clients
• Caller-injected clients must remain caller-owned and must not be silently reconfigured or closed by the library
• Library-created clients must support the package User-Agent and library-owned lifecycle rules
• Retry behavior may be implemented by the library; the dependency does not need an urllib3.Retry equivalent
• Underlying transport-specific exceptions and types must remain implementation details rather than public API commitments

The optional dependency choice should not force changes to the synchronous Mlb or MlbDataAdapter API.

Constraints

• Existing synchronous users should not receive an unnecessary async HTTP dependency
• Do not change the existing runtime dependency contract unless required
• Error messages for a missing async extra should be actionable
• Do not expose the selected transport library as part of the stable mlbstatsapi public API

Acceptance criteria

• Normal install works without async dependency
• [async] extra installs required async dependency
• Missing-extra behavior is clear and tested
• Wheel/sdist metadata contains the optional extra correctly
• Existing sync package imports remain unaffected
• Selected dependency supports the lifecycle, timeout, cancellation, ownership, and same-event-loop concurrency requirements from Define the v1.1 async API and transport contract #298

Refs #297
Contract: #298