feat: add lazy connection configuration option

[zaawee]

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
• 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?

The current behavior of the module is that when the connection is injected it also connects to the database. This is caused by the use of .asPromise() when creating the connection.

This caused issues for us when we tried starting a service without having mongo up and running.

Issue Number: N/A

What is the new behavior?

This PR adds a configuration option for the module that enables the use of the module without it connecting automatically when the connection is injected.

The new config interface is:

export interface MongooseModuleOptions
  extends ConnectOptions,
    Record<string, any> {
  uri?: string;
  retryAttempts?: number;
  retryDelay?: number;
  connectionName?: string;
  connectionFactory?: (connection: any, name: string) => any;
  connectionErrorFactory?: (error: MongooseError) => MongooseError;
  lazyConnection?: boolean;
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

[zaawee]

Hey @kamilmysliwiec,
This PR has been sitting here for quite a while, and I'm wondering if there's anything I should fix or do in order to help this merge
Thanks

[kamilmysliwiec]

lgtm