Sometimes called "fake sign" or "OSS sign" public signing is including
the public key in an output assembly and setting the "signed" flag, but
not actually signing the assembly with a private key. This is useful for
open source projects where people want to build assemblies which are
compatible with the released "fully signed" assemblies, but don't have
access to the private key used to sign the assemblies. Since almost no
consumers actually need to check if the assembly is fully signed, these
publicly built assemblies are useable in almost every scenario that the
fully signed one would be used in.

This PR implements support only for C# -- VB will be added soon. If
being used at the command line, the /publicsign flag can be passed to
csc and the /keyfile flag can specify the public key. Unlike fully
signing, a full key pair encoded in the SNK file format is not currently
supported. When using /publicsign, just the public key must be in the
/keyfile file.

When using the API, the public key can be passed directly using the
CryptoPublicKey CompilationOption.