Add proto definitions for (AES) KeyVersion.

Also removing placeholders, since now we have a complete set of code, tests and protos.
This is the key material - the raw bytes of the AES key. I used the same
field name as the current AesKeyVersion implementation. Should we change it?

On Saturday, July 5, 2014, Shawn Willden <notifications@github.com> wrote:

> In protos/aes_key_version_proto.proto:
>
> > +
> > +package com.google.k2crypto.keyversions;
> > +
> > +message AesKeyVersionData {
> > +  extend KeyVersionData {
> > +    required AesKeyVersionData keyVersion = 10001;
> > +  }
> > +  // Non-core fields go here
> > +}
> > +
> > +message AesKeyVersionCore {
> > +  extend KeyVersionCore {
> > +    required AesKeyVersionCore keyVersion = 10001;
> > +  }
> > +  // Core fields will be hashed into the KeyVersion ID
> > +  optional bytes matter = 1;
>
> What is "matter"?
>
> —
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> <https://github.com/google/K2/pull/20/files#r14572340>.
>