Pass backend_options to Trio

[Zac-HD]

Changes

In AnyIO 3.x the module-level run = trio.run was invoked as asynclib.run(func, *args, **backend_options), but since 4.0's AsyncBackend refactor anyio.run(..., backend="trio", backend_options={...}) silently discards the backend_options.

We still document trio backend_options as forwarded to trio.run, with restrict_keyboard_interrupt_to_checkpoints as an example, so this PR restores support in the obvious way.

A minimal motivating repro:

import time, trio, trio.testing
from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()
@app.get("/s")
async def s():
    await trio.sleep(5)
    return {"t": trio.current_time()}

c = TestClient(app, backend="trio",
               backend_options={"clock": trio.testing.MockClock(autojump_threshold=0)})
t0 = time.monotonic()
c.get("/s")
print(f"wall={time.monotonic()-t0:.2f}s")  # 5.01s — clock never reached trio.run

Fixes #1162.

Checklist

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (n/a; restoring already-documented behavior)
• You've added a new changelog entry (in docs/versionhistory.rst).

[agronholm]

LGTM.