fix(): fix multiple document not working when using fastify

[Boshen]

fixes #234
fixes fastify/fastify-swagger#213

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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[*] No

Other information

[Boshen]

see fastify/fastify-swagger#216 (review)
I've tested it in my own project

[nartc]

what happens here?

[Boshen]

Apparently fastify plugin works differently. Per its documentation, this adds a different scope so the internal call
fastify.decorate('swagger', swagger) does not collide when we try to make multiple swagger docs

[kamilmysliwiec]

Can you create an integration test for this change too?

[Boshen]

Can you create an integration test for this change too?

@kamilmysliwiec Done!

[Boshen]

@kamilmysliwiec can you clean this up and merge this? I really want this for my project, thanks!

[kamilmysliwiec]

Thank you @Boshen!

[lock]

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