feat: allow use as a singleton

This might make things even easier to get started?

```TypeScript
import { verifiedFetch } from '@helia/verified-fetch'

const response = await verifiedFetch('ipfs://Qmfoo')

console.info(await response.json())
```

It's not in this PR but maybe we'd want to allow some config:

```TypeScript
import { verifiedFetch } from '@helia/verified-fetch'

const response = await verifiedFetch('ipfs://Qmfoo', {
  gateways: ['https://...']
})

console.info(await response.json())
```

Calling this twice with different gateways might result in an error.

```TypeScript
import { verifiedFetch } from '@helia/verified-fetch'

const response1 = await verifiedFetch('ipfs://Qmfoo', {
  gateways: ['https://foo...']
})

const response2 = await verifiedFetch('ipfs://Qmfoo', {
  gateways: ['https://bar...']
})
// Error: Only one set of gateways/routers may be specified, please use `createVerifiedFetch` for more flexibility
```

packages/verified-fetch/src/index.ts

@@ -300,6 +300,8 @@ export async function createVerifiedFetch (init?: Helia | CreateVerifiedFetchWit
   return verifiedFetch
 }
 
+export { verifiedFetch } from './singleton.js'
+
 function isHelia (obj: any): obj is Helia {
   // test for the presence of known Helia properties, return a boolean value
   return obj?.blockstore != null &&

packages/verified-fetch/src/singleton.ts

@@ -0,0 +1,23 @@
+import { createVerifiedFetch } from './index.js'
+import type { Resource, VerifiedFetch, VerifiedFetchInit } from './index.js'
+
+interface VerifiedFetchSingleton extends VerifiedFetch {
+  _impl?: VerifiedFetch
+}
+
+const singleton: VerifiedFetchSingleton = async function verifiedFetch (resource: Resource, options?: VerifiedFetchInit): Promise<Response> {
+  if (singleton._impl == null) {
+    singleton._impl = await createVerifiedFetch()
+  }
+
+  return singleton._impl(resource, options)
+}
+singleton.start = async function () {
+  await singleton._impl?.stop()
+}
+singleton.stop = async function () {
+  await singleton._impl?.stop()
+}
+const verifiedFetchSingleton: VerifiedFetch = singleton
+
+export { verifiedFetchSingleton as verifiedFetch }

packages/verified-fetch/test/index.spec.ts

@@ -2,7 +2,7 @@
 import { createHeliaHTTP } from '@helia/http'
 import { expect } from 'aegir/chai'
 import { createHelia } from 'helia'
-import { createVerifiedFetch } from '../src/index.js'
+import { createVerifiedFetch, verifiedFetch } from '../src/index.js'
 
 describe('createVerifiedFetch', () => {
   it('can be constructed with a HeliaHttp instance', async () => {
@@ -45,4 +45,10 @@ describe('createVerifiedFetch', () => {
     expect(verifiedFetch).to.be.ok()
     await verifiedFetch.stop()
   })
+
+  it('can be used as a singleton', () => {
+    expect(verifiedFetch).to.be.a('function')
+    expect(verifiedFetch.stop).to.be.a('function')
+    expect(verifiedFetch.start).to.be.a('function')
+  })
 })