feat: add per route busboy configuration

[grigoran]

There was a need to restrict the file size in each route when used alongside attachFieldsToBody. These changes will enable route-specific busboy configuration.
Example

fastify.post('/', {
    schema: {
        consumes: ['multipart/form-data'],
        body: Type.Object({
            // file: {isFile: true} as unknown as TSchema
            file: Type.Unsafe<{ toBuffer: () => Promise<Buffer> }>({ isFile: true })
        }),
        response: {
            200: Type.Object({
                ok: Type.Boolean()
            })
        }
    },
    config: {
        multipart_options: {
            limits: {
                fileSize: 1024*1024*2 // 2MB
            }
        }
    }
}, async (req, rep) => {
    const buf = req.body.file.toBuffer();
    return { ok: true }
})

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

[Eomm]

Could you add some unit test?

[grigoran]

Thank you! Done