feat(microservices): added MqttOption for passing raw output to pub

[notusertelken]

PR Checklist

Please check if your PR fulfills the following requirements:

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

mqtt-server and client stringify the output of the serializer before emitting/publishing events

Issue Number: 7288

What is the new behavior?

moved JSON.stringify to the serializer, so now the output of the serializer goes directly into publish/emit.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 6fa77dde-eb4c-4a5b-aab8-98160be3959b

• 14 of 14 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 94.135%

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Totals
Change from base Build 030c7bfc-7fd6-457a-8c24-3f90c6cb9edf:	0.005%
Covered Lines:	5698
Relevant Lines:	6053

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💛 - Coveralls

[notusertelken]

If this implementation is found acceptable, ill shortly make PRs for the other transport servers

[kamilmysliwiec]

@notusertelken looks good!

[notusertelken]

had forgotten about the client :sweat_smile:

[Clex1o1]

Here is also a string transformation of the payload. Shouldn't it be possible to use raw format too? https://github.com/nestjs/nest/blob/cdf2aa5b067386677fc6b97f934acd7b18da3dcc/packages/microservices/server/server-mqtt.ts#L102