refactor(platform-fastify): use the route method to inject routes

[johaven]

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

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?

Route injection is handled by the shorthands methods

What is the new behavior?

As discussed here: fastify/fastify#5412 (comment)

Fastify allows the user to decorate the instance with any name. This can cause problems if common words are used instead of HTTP methods.

The other advantage is that Nest will not depend on the implementation of shorthands methods on the fastify side.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Related to #13278

[coveralls]

Pull Request Test Coverage Report for Build 04022db3-2466-4895-8ac9-4f29d9445079

Details

• 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 92.124%

---

Totals
Change from base Build b32e22d6-f72e-486a-85a2-044c2b5df08f:	0.0%
Covered Lines:	6737
Relevant Lines:	7313

---

💛 - Coveralls