Add CreateSymlink to storage provider

[butonic]

In order to transparently handle symlinks the cs3api should have a dedicated call to create symlinks. We plan to use it so that our settings, accounts and store service can use the cs3 api to persist data.

[labkode]

@butonic I need more context on what you're trying to achieve

[butonic]

Currently, building a monotonic increasing counter on top of a distributed filesystem using symlinks (or file locks, tracked in #6).

But we also have in mind being able to sync symlinks with the desktop client. owncloud/client#1440 as have you owncloud/ocis#43

[labkode]

@butonic that I understand, but not the relationship with "settings, accounts and store service"

[butonic]

currently they persist data in a local folder. but for HA and horizontal scalability we want to use the cs3 api to persist data.

[butonic]

supporting symlinks is not a top priority for the owncloud sync, but we would like to use them with services in ocis. instead of relying on a local storage we would point them to a namespace in the cs3api and could use the storage layer underneath it.