feat(microservices): Add producer reference to KafkaContext

[Papooch]

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)
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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?

Kafka producer is not exposed in KafkaContext

Issue Number: #10263

What is the new behavior?

Kafka producer is exposed in KafkaContext under getProducer()
Closes #10263

Does this PR introduce a breaking change?

• Yes
• No

In order not to introduce a braking change, I added the producer as the last argument of KafkaContextArgs. It would look better if it were immediately after consumer.

Other information

Btw, I noticed that all Kafka related integration tests are skipped for being "flaky", therefore I didn't add one. Is this something that can be worked on to make them reliable?

[coveralls]

Pull Request Test Coverage Report for Build fd0166ec-e486-472a-b3c5-0f6be57f59b8

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 93.818%

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Totals
Change from base Build e5cc639c-5942-49dc-b6e6-87bb2881b30e:	0.001%
Covered Lines:	6101
Relevant Lines:	6503

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💛 - Coveralls

[kamilmysliwiec]

lgtm