Add parser middleware to /generate

[Amzani]

Why do we need this improvement?

After the merge #1200, we have the/generate API endpoint, but it missed the parser middleware.

How will this change help?

Validate AsyncAPI files documents before calling the generator

Screenshots

No response

How could it be implemented/designed?

Use the middleware, see https://github.com/asyncapi/server-api

🚧 Breaking changes

No

👀 Have you checked for similar open issues?

• I checked and didn't find a similar issue

🏢 Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

Are you willing to work on this issue?

Yes I am willing to submit a PR!

[Amzani]

No relevant for now