feat(fastify): add decorator for custom schema

[piotrfrankowski]

Add an ability to attach json schema to request with a RequestSchema decorator

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?

Currently, there is no easy way of using fastify's schema property and adding a response schema can have positive impact on performance source

Issue Number: N/A

What is the new behavior?

This PR adds a new decorator RequestSchema that takes a schema object and sets it as schema property on fastify route doc

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build 63f579de-c26f-456f-827f-1ddd0f85fdbe

Details

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 89.298%

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Totals
Change from base Build 44c104f1-8e85-45d8-85b0-095b2f828590:	0.006%
Covered Lines:	7151
Relevant Lines:	8008

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💛 - Coveralls