feat: let ConfigService type know about schema validation

[micalevisk]

PR Checklist

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• 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: #668

What is the new behavior?

Adds a second optional type arg to ConfigService generics.

Users now can tell to ConfigService that some validation was made on ConfigModule#forRoot, so there's no room for ConfigService#get to return undefined when we narrow it to some type like:

// ...
// after making sure that you set up `ConfigModule#forRoot` with `validationSchema`
// ...

interface EnvironmentVariables {
  PORT: number;
  TIMEOUT: string;
}

// ...
constructor(private configService: ConfigService<EnvironmentVariables, true>) {
  //                                                                   ^^^^ Telling to ConfigService
  //                                                                        that everything was validated
  const port = this.configService.get('PORT');
  //    ^^^ the type of this cannot be `undefined` anymore, just `number`
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

I could update the https://docs.nestjs.com/techniques/configuration after merging this PR

[micalevisk]

I'm not sure if this strategy is that bad tho. Since I don't want to add libs such as tsd or dtslint to check if my proposing feature for ConfigService is working as expected (and will keep working in the future), I've added this method to do the type checking as a side-effect of running npm run test:integration :p

I could drop this commit if you want to. Or I could add one more to test the this.configServiceNarrowed.get(x) use case (ie., relying on type inference)

[kamilmysliwiec]

LGTM