Create Hudl.Mjolnir.Attributes package

This package will replace the command attributes included in Mjolnir proper.

Internally, we use these attributes in lightweight client packages, and we need those packages to have as few dependencies as possible to reduce coupling and transitive dependency resolution when they're imported into other projects.

Once this is pushed, I'll update Mjolnir to accommodate these attributes. The ones inside Mjolnir will become `[Obsolete]`, but will continue to work until the next major release.