feat: better generic support for registerAs

[JasonHK]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

This is how to specify the return type of a config factory before this commit:

import { registerAs } from '@nestjs/config';
import { ConfigFactory } from '@nestjs/config/dist/interfaces';

registerAs<ConfigFactory<DatabaseConfig>>('database', () => ({
  host: 'host',
  port: 4000,
}));

interface DatabaseConfig {
  host: string;
  port: number;
}

What is the new behavior?

Better support to specify the return type of a config factory.

import { registerAs } from '@nestjs/config';

registerAs<DatabaseConfig>('database', () => ({
  host: 'host',
  port: 4000,
}));

interface DatabaseConfig {
  host: string;
  port: number;
}

Does this PR introduce a breaking change?

[x] Yes
[ ] No

The commit will break the code which already utilizing the generic of
registerAs function.

Other information

[JasonHK]

@kamilmysliwiec The PR should be able to be merged now. Could you please review it?

[kamilmysliwiec]

@JasonHK PR looks great, but it introduces a breaking change which means that we can't really merge it till the next major release.