Add an experimental GCSDownloader for HTTPS Collection support

[sirosen]

This is an initial experimental implementation, not necessarily our long-term interface.
It solves the most important problem in handling HTTPS for GCS, which is translating a file URL into a collection ID.

The interface is the part most subject to potential future refinement, as it's not clear how to provide the right level of control over

• the transport used to sniff details
• the transfer client used to lookup by ID
• the client and transport used to actually do the download

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This is an implementation of HTTPS download from GCS collections via
Location header sniffing, which is the only mechanism we have available
on all GCS versions for translating a URL to a client ID.