feat(content-type): add strictContentTypeHeaderValidation option - #6926

I reworked #6918 into an option.

Adds a default-on strictContentTypeHeaderValidation server option for applications that must accept known and unknown malformed Content-Type request headers from clients they cannot control. It basically adds an option to switch lib/handle-request.js back to not validating before consulting the array of registered content type header handlers.

Behavior

Default behavior is unchanged:

• invalid Content-Type header values are rejected with 415 FST_ERR_CTP_INVALID_MEDIA_TYPE
• no body parser is consulted

With strictContentTypeHeaderValidation: false:

• RegExp parsers can match the raw malformed header value
• a catch-all parser can handle the raw malformed header value
• an explicit parser is still required; otherwise Fastify returns 415
• string parsers remain valid-media-type registrations only
• malformed input preserves the existing request.mediaType behavior ("/")
• schema.body.content cannot select a narrowed validator for malformed input

Checklist

• run npm run test && npm run benchmark --if-present
• 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