feat(functions): add async function invocation

[Abhijeet Prasad (AbhiPrasad)]

resolves https://linear.app/braintrustdata/issue/SDK-140/support-async-version-if-invoke-in-python-sdk

Add invoke_async() with the same configuration and typed stream overloads as invoke(). Non-streaming calls return the decoded result without blocking the event loop; streaming calls return BraintrustStream with async iteration and final_value_async() support.

stream=False is the default. The complete decoded result is returned:

from braintrust import invoke_async

result = await invoke_async(
    project_name="my-project",
    slug="my-function",
    input={"question": "What is 2 + 2?"},
)

async task
    │ await
    ▼
invoke_async()
    │ worker thread
    ▼
invoke() ──HTTP──► Braintrust
    │
    ▼
decoded result

Set stream=True to receive a BraintrustStream supporting async for:

stream = await invoke_async(
    project_name="my-project",
    slug="my-function",
    input={"question": "Tell me a story"},
    stream=True,
)

async for chunk in stream:
    print(chunk)

invoke_async(stream=True)
    │ worker opens SSE request
    ▼
BraintrustStream
    │
    ├── async for
    │      └── worker reads each SSE chunk
    │
    └── await final_value_async()
           └── worker aggregates the stream