Create HealthIndicatorService to simplify custom health indicators

[BrunnerLivio]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/terminus/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
• 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?

Currently, a custom health indicator looks as follows:

@Injectable()
export class DogHealthIndicator extends HealthIndicator {
  constructor(private readonly dogService: DogService) {
    super();
  }

  async isHealthy(key: string): Promise<HealthIndicatorResult> {
    const dogs = await this.dogService.getDogs();
    const badboys = dogs.filter((dog) => dog.state === DogState.BAD_BOY);
    const isHealthy = badboys.length === 0;

    const result = this.getStatus(key, isHealthy, { badboys: badboys.length });

    if (isHealthy) {
      return result;
    }
    throw new HealthCheckError('Dog check failed', result);
  }
}

Terminus provides an API that falls short on the following points:

• Uses inheritance over composition
• Makes testing more difficult
• The HealthIndicator.getStatus function has an undescriptive name and a difficult-to-read API interface

A new way to construct custom health indicator would be proposed in order to resolve these issues:

@Injectable()
export class DogHealthIndicator {
  constructor(
    private readonly dogService: DogService,
    private readonly healthIndicatorService: HealthIndicatorService,
  ) {}

  async isHealthy<const TKey extends string>(key: TKey) {
    const indicator = this.healthIndicatorService.check(key);

    const dogs = await this.dogService.getDogs();
    const badboys = dogs.filter((dog) => dog.state === DogState.BAD_BOY);
    const isHealthy = badboys.length === 0;

    if (!isHealthy) {
      return indicator.down({
        badboys: badboys.length,
      });
    }

    return indicator.up();
  }
}

What is the new behavior?

Simple, more readable code

Does this PR introduce a breaking change?

• Yes
• No => A deprecation warning will be logged, though the old API will still work as is