Consult remote substituters for realizations and store objects during build

[zombiezen]

• Add zb serve configuration option for remote substituters. I suspect this should be in a dedicated "serve" section of the configuration, rather than command-line flags, since there can be complex details like credential files etc.
• If there are no suitable local realizations or store objects during a build, search in remote substituters.
• For realizations, add all realization data unconditionally. It's lightweight, and can be consulted later to skip straight to store object downloads. Helps discover nondeterministic realizations.
• For store objects, we can treat the download like an import. We should verify the store object contents on the way in for integrity as normal.