Limit the exposed API to necessary methods and types

Since we're working on a v3 release, it might be wise to revise the API of the library. Multiple types are exposed that seem not necessary, which might result in issues when internal structure of the library is modified (e.g. `LegacyEvent`).

It's also easier for developers when autocomplete suggests only usable types and it's not bloated with internals.

We should verify that all exported types actually belong to the API layer. This can be done by modifying the following:

https://github.com/firebase/firebase-functions/blob/18c01cbdc25d8f4fccb8847f6a4be86b6c7d4960/src/index.ts#L55-L57

in the following way:

```ts
export { Only, Necessary, Types } from './config';
export { No, Internals } from './cloud-functions';
export { Nice, And, Small } from './function-builder';
```