FR: `rules-unit-testing v2` expose `RulesTestContext.functions()`

### Describe your environment

* Operating System version: MacOS 12.1
* Browser version: Firefox 97.0.1
* Firebase SDK version: @firebase/rules-unit-testing 2.0.1
* Firebase Product: rules-unit-testing, functions

### Describe the problem

I would like to write automated e2e tests to validate my cloud functions using Firebase emulators.

With `@firebase/rules-unit-testing v1`, I could easily get a `Functions` context with fake authentication :
```ts
const app = initializeTestApp({
projectId: MY_PROJECT_ID,
auth: { uid: "my_uid"}
})
app.functions().useEmulator("localhost", 5001)
const funcToTest = app.functions().httpsCallable("myFunctionToTest")
// Do some tests
```

With `@firebase/rules-unit-testing v2`, there is no way to do the same :

```ts
const testEnv = await initializeTestEnvironment({
projectId: MY_PROJECT_ID,
})

const authenticatedContext = testEnv.authenticatedContext("my_uid")

//There is no `authenticatedContext.functions()` accessor
```

This seems extremely problematic to me, since it makes cloud functions e2e testing a real pain.

I would be happy to talk about this with you :)