Add "ECDSA using curve P-384 DSS and SHA-384" algorithm

[richanna]

Define a signature algorithm named ecdsa-p384-sha384 that indicates the use of ECDSA using curve P-384 DSS and SHA-384. The algorithm description is probably just 3.3.4 with "256" replaced with "384", but should validate that assumption with actual crypto folks.

[jricher]

What's the push for this specific curve to be included here? The reason I ask is that we shouldn't necessarily try to define ALL known algorithms in here, when it's extensible.

[richanna]

I agree with not defining every possible algorithm; however we should include those with known use cases, and AWS intends to use -384. Since the spec isn't finalized and the addition is trivial, it seems reasonable to add it here rather than punting it to an extension spec.