`*` in arbitrary _metadata_keys may need clarification

[butonic]

When implementing initial checksum support I stumbled over a difference with the webdav api that may need clarification in eg.: https://github.com/cs3org/cs3apis/blob/8a19a7f60664dbcff80bbe46348b30562e16abbc/cs3/storage/provider/v1beta1/provider_api.proto#L337-L340

We currently translate an allprops PROPFIND to a * key, but allprops should only return some default properties. Some live properties might be expensive to compute so they are not calculated in an allprops PROPFIND. See https://tools.ietf.org/html/rfc4918#section-9.1