Implemented a new function to register a GatewayServiceClient. Previously, this could only be done by passing an actual connection. However, that is unnecessarily limiting. Introducing the RegisterGatewayServiceClient allows us to register mocked gateways for unit testing

This commit introduces a new header, `X-Disable-Versioning`, that is available on PUT requests. This header will disable versioning for this file save if the storage backend is EOS.

Additionally, this commit introduces two unit tests related to this functionality:
- TestDisableVersioningLeadsToCorrectQueryParams: test whether disabling versioning leads to correct query parameteers for the EOS HTTP / GRPC client
- TestDisableVersioningHeaderPassedAlong: test whether a header passed to the initial endpoint is propagated to the actual upload endpoint

Finally, this commit also fixes a bug that was already present, where
the app was not passed along in the case of token-based authz