fix(params): exporting additional type information

[vajahath]

Description

As per this doc, I've been using the defineSecret function. The return type SecretParam was not available for direct use.

import { SecretParam } from 'firebase-functions/params'; // fails
import { SecretParam } from 'firebase-functions/params/types.d'; // fails

This PR fixes it.

Code sample

With this PR we can do the following:

import { SecretParam } from 'firebase-functions/params'

[vajahath]

Current solution without this PR:

type SecretParameter = ReturnType<typeof defineSecret>;

[YannicEl]

Was about to make the same PR. This would be really nice to have. Any chance this could be merged?

[exaby73]

Closing this PR based on https://github.com/firebase/firebase-functions/issues/1549#issuecomment-2054135491