Extend @exposed decorator to simplify and generalize recurring code checks

[phorward]

This issue is a replacement for #618 to collect several scenarios and use-cases where an extended @exposed-decorator can be useful to configure a function's behavior, resulting in secure, less error prone and shorter code.

restrict

To avoid function definitions like this:

    @exposed
    def do_something(self, action: str):
        if not ((cuser := current.user.get()) and "root" in cuser["access"]):
            raise errors.Unauthorized()

        # ...

a restrict-attribute can be used like this:

    @exposed(restrict="root")
    def do_something(self, action: str):
        # ...

or when thinking further, using a lambda:

    @exposed(restrict=lambda user: "root" in user["access"] or "admin" in user["access"])
    def do_something(self, action: str):
        # ...

response

To avoid function definitions like this:

    @exposed
    def do_something(self, action: str):
        current.request.get().response.headers["Content-Type"] = "application/json"

        match action:
            # ...

            case _:
                raise errors.NotImplemented(f"Action {action!r} not implemented")

        return json.dumps("OKAY")

a response-attribute could be used like this.

    @exposed(response="json")
    def do_something(self, action: str):
        match action:
            # ...

            case _:
                raise errors.NotImplemented(f"Action {action!r} not implemented")

        return "OKAY"

skey

To avoid function definitions like this:

    @exposed
    def do_something(self, action: str, skey: str):
        if not securitykey.validate(skey):
            raise errors.PreconditionFailed()
        # ...

an skey-attribute can be used,

    @exposed(skey=True)
    def do_something(self, action: str):
        # ...

optionally with an lambda, evaluating the result of the securitykey.validate() function

    @exposed(skey=lambda skey: skey["user"] == current.user.get()["key"])
    def do_something(self, action: str):
        # ...

[sveneberth]

In the ViUR meeting we decided to use multiple single decorators in snake_case.

[phorward]

Implemented by #800