docs: add registerAsync to microservices client section

[vahidvdn]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

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
• [x] Docs
• [ ] Other... Please describe:

What is the current behavior?

There is no info for registerAsync which is more useful than register method in real applications

Issue Number: N/A

What is the new behavior?

Add an example of registerAsync method.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[vahidvdn]

@kamilmysliwiec Any plan to merge this?