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Added support for remote pinning services

A pinning service is a service that accepts CIDs from a user in order to host the data associated with them.
The spec for these services is defined at https://github.com/ipfs/pinning-services-api-spec

Support is available via the `ipfs pin remote` CLI and the corresponding HTTP API
Co-authored-by: Petar Maymounkov <petarm@gmail.com>
Co-authored-by: Marcin Rataj <lidel@lidel.org>
Co-authored-by: Adin Schmahmann <adin.schmahmann@gmail.com>