Make json body parser limit configurable

[Peter Sutter]

What would you like to be added:
Currently, if you attempt to modify a shoot resource that is significantly large (for example, one that has many worker group configurations), you may encounter a failure. The request could fail and return the following error:

{
    "code": 413,
    "reason": "Payload Too Large",
    "message": "request entity too large",
    "status": "Failure",
    "details": {
        "name": "PayloadTooLargeError"
    }
}

To accommodate larger resource sizes, the JSON body parser limit should be made configurable. For more information, refer to the Express.js API documentation.

Why is this needed: