This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
When wrapping a backend that supports Server Side Copy (e.g. `b2`, `s3`)
and configuring the `tmp_upload_path` option, the `cache` backend would
erroneously report that Server Side Copy/Move was not supported, causing
operations such as file moves to fail. This change fixes this issue
under these circumstances such that Server Side Copy will now be used
when the wrapped backend supports it.
Fixes#3206
These commands are for implementing backend specific
functionality. They have documentation which is placed automatically
into the backend doc.
There is a simple test for the feature in the backend tests.
