`*` 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.:

cs3apis/cs3/storage/provider/v1beta1/provider_api.proto

Lines 337 to 340 in 8a19a7f

	// OPTIONAL.
	// Arbitrary metadata be included with the resource.
	// A key with the name '*' means to return all available arbitrary metadata.
	repeated string arbitrary_metadata_keys = 3;

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