fix(microservices): Fixed typings for MessageHandler

[dkonasov]

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
• 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: #9668

What is the new behavior?

MessageHandler return type was changed to Promise<Observable<TResult>> | Promise<TResult>

Does this PR introduce a breaking change?

• Yes
• No

Since the return type had changed, users may be required to cast type explicitly in the old code.

let handler: MessageHandler;
...
let result = await messageHandler(data);
// Here we need type casting
(result as Observable<...>).pipe(...);
// Same for next method
result = await messageHandler.next(data);
(result as Observable<...>).pipe(...);

Other information

fixes #9668

[coveralls]

Pull Request Test Coverage Report for Build b3c7a6d9-e836-4faf-88c7-911067867cbc

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.107%

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Totals
Change from base Build 0b0f62d7-5768-434b-b26f-340c513ce1c1:	0.0%
Covered Lines:	5781
Relevant Lines:	6143

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💛 - Coveralls