Access collections methods directly from cozy-client API

[paultranvan]

For now, some methods defined in collections are only accessible by calling directly calling the related collection, e.g. client.collection('io.cozy.files').updateMetadataAttribute(...).

This is not ideal, because:

• It forces the developers to know many API methods
• It bypasses the cozy-client logic, and notably the store.

See this PR discussion for a first work and discussion about it: #650

Also related to #244