Optional receiver_key to enable encryption of SETs

[FragLegs]

• Added optional receiver_key to enable encryption of SETs
• Added language in Security Considerations section to describe when SETs MUST be encrypted
• Added language in Security Considerations offering alternative methods of sharing the receiver's public key, including Dynamic Client Registration

During the WG meeting, we discussed using Dynamic Client Registration out-of-band to share the Receiver's public key with the Transmitter. However, that forces the Transmitter to use OAuth2, which we have intentionally tried to avoid in the spec. So instead I provided an optional in-spec method for sharing the Receiver's public key. For those who would prefer to use Dynamic Client Registration, I also added language to the Security Considerations section describing that as a potential option for secret sharing.

Fixes Issue #140