This enables using for http for S3 request for debugging or
implementations that don't have https configured. This is not a problem
for binary caches since they should not contain sensitive information.
Both package signatures and AWS auth already protect against tampering.
This allows specifying the AWS configuration profile to use. E.g.
nix copy --from s3://my-cache?profile=aws-dev-account /nix/store/cf3isrlqavvd5w7rpky1fa8j9lcnlggm-...
This adds support for s3:// URIs in all places where Nix allows URIs,
e.g. in builtins.fetchurl, builtins.fetchTarball, <nix/fetchurl.nix>
and NIX_PATH. It allows fetching resources from private S3 buckets,
using credentials obtained from the standard places (i.e. AWS_*
environment variables, ~/.aws/credentials and the EC2 metadata
server). This may not be super-useful in general, but since we already
depend on aws-sdk-cpp, it's a cheap feature to add.
