fix: add content type parsers after removeAllContentTypeParsers

[realtimetodie]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Fixes an issue where it was not possible to add content type parsers after calling removeAllContentTypeParsers.

Example

fastify.removeAllContentTypeParsers()

fastify.addContentTypeParser('text/xml', function (request, payload, done) {
  xmlParser(payload, function (err, body) {
    done(err, body)
  })
})

Documentation

• https://github.com/fastify/fastify/blob/main/docs/ContentTypeParser.md#removeallcontenttypeparsers

[mcollina]

lgtm

[realtimetodie]

added more tests

[github-actions]

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