Fixing up Attested Operations

[Kelley Burgin <kwburgi@tycho.ncsc.mil>]

To address Bruce's comment on Attested Operations that the server  
should indicate which attestation methods it will accept instead of  
making the client guess, here are a few choices:

1. The client sends a list of supported attestation types in the  
header of its requests instead of the "Attestation Capable Indicator".  
If the client does not include an Attestation Credential object, the  
server responds with an "Attestation Required" error message that  
includes a nonce object and list of supported attestation types in the  
header.

2. Add a new operation (Discover Attestation Types?) similar to  
Discover Versions where the client can send an ordered list of  
supported attestation types and the server responds with its ordered  
list of acceptable attestation types.

3. Add a new Query Function (Query Attestation Types?) to Query that  
the server responds to with its list of supported attestation types.

What do you think?

Kelley