support Signed-By in Release files as a sort of HPKP

support Signed-By in Release files as a sort of HPKP

Users have the option since apt >= 1.1 to enforce that a Release file is
signed with specific key(s) either via keyring filename or fingerprints.
This commit adds an entry with the same name and value (except that it
doesn't accept filenames for obvious reasons) to the Release file so
that the repository owner can set a default value for this setting
effecting the *next* Release file, not the current one, which provides a
functionality similar "HTTP Public Key Pinning". The pinning is in
effect as long as the (then old) Release file is considered valid, but
it is also ignored if the Release file has no Valid-Until at all.