Custom hook to handle mime type interpretation

[JeanJPNM]

Is your feature request related to a problem? Please describe.
Possible solution for #1438, better thought of and more maintainable than #1687.

Describe the solution you'd like
A custom hook that lets the user determine how the body of a request/response should be parsed based on its mime type.
The hook receives the mime type or the request/response headers as input and returns a value from an enum, telling the framework how to interpret the body.

Describe alternatives you've considered

• Two fields in the svelte.config.js file (one for buffer and other for text) that contain a list of regexes that are tested against a mime type to determine how the body should be parsed.
• Hardcoded list of mime types for body parsing.

How important is this feature to you?
For me is a very important feature since it will enable users to send binary content from endpoints, and let them define specific handling for some mime types such as image/svg+xml.

Additional context
This feature requires svelte kit to make hooks accessible to adapters.
This feature was made considering that #1563 may become a part of the framework hence the use of an enum instead of a boolean.
This feature is also intended to replace #1687.

[JeanJPNM]

Here is a quick and working implementation, will be converted to a draft or pull request if this issue is considered relevant

[benmccann]

@JeanJPNM do you think we still need this after #1890 and #1900?

[JeanJPNM]

No I don't think this is necessary anymore, since the the body will default to an instance of Uint8Array instead of a string it is pretty easy to parse it inside an endpoint.