Feature Request: Support `waitUntil` in middleware for background task scheduling

Summary

When building serverless applications with Hono on Cloudflare Workers or similar edge runtimes, it's common to need waitUntil() from the execution context to fire-and-forget background tasks (e.g. logging, analytics, cache warming) without blocking the response.

Problem

Currently, accessing ctx.executionCtx.waitUntil() inside Hono middleware requires either:

1. Passing the execution context manually through every handler
2. Using a workaround via c.env to store it

There's no idiomatic, first-class way to use waitUntil in middleware.

Proposed Solution

Expose a c.waitUntil(promise) helper on the Hono Context object, similar to how c.executionCtx already works but with a more ergonomic interface:

app.use(async (c, next) => {
  await next()
  // Fire-and-forget: log request after response is sent
  c.waitUntil(logRequest(c.req.raw))
})

Why This Matters

• Cloudflare Workers, Deno Deploy, and Vercel Edge all support waitUntil semantics
• Frameworks like Remix and Next.js are adding similar patterns
• Makes it easier to add observability (logging, tracing) without impacting response latency

Additional Context

• Related: https://developers.cloudflare.com/workers/runtime-apis/context/#waituntil
• Hono already has c.executionCtx — this would be a thin ergonomic wrapper

Would love to hear if there's a reason this hasn't been added, or if there's a recommended pattern I'm missing!