Allow handling multiple checksums

[butonic]

• The storage providers ResourceInfo should be able to return more than one checksum.

• We need a way to let clients specify which checksums they are requesting. In initial checksum support for ocis reva#1400 I used a checksum metadata key in the Stat and ListContainer to indicate I want to read all checksums. Do we want to be able to specify which algorithm? Using sth like checksum:sha1, checksum:md5, checksum:sha1,md5,adler32 similar to HTTP headers? ure use a format similar to what TUS is using ([algo] [hash])?

• InitiateFileUploadRequest needs a way to pass the expected checksum. For both implementing OC 1- style checksums as well as the TUS checksumming extension. In initial checksum support for ocis reva#1400 I am using an "Upload-Checksum" in the Opaque properties of the request: https://github.com/cs3org/reva/pull/1400/files#diff-198f1004a921b3627f7572a239452974429a7de6e4fa47f445c2ad35d2cd9026R320