Add secp256r1 signature support

[aaronc]

Also known as NIST P-256. See cosmos#4789

[aaronc]

Note support for this is already in the go crypto library as Zaki mentioned: https://golang.org/src/crypto/elliptic/elliptic.go?s=11372:11389#L368

[aaronc]

@sahith-narahari is this something you could work on? Let's discuss.

[sahith-narahari]

We shall proceed by implementing pubkey interface for nist p-256. Future steps would be to figure a way to get address from pubkey.
To be discussed is how we can get this implementation for encoding either using protobuf directly or amino.

Useful links

https://golang.org/pkg/crypto/elliptic/#P256
https://golang.org/pkg/crypto/ecdsa/

[clevinson]

@aaronc please add me to the key mgmt group so I can track this ticket in codetree.

[clevinson]

@sahith-narahari can you tag @aaronc as a reviewer in a WIP PR so we can take a look at this?

[sahith-narahari]

Sure @clevinson , will raise a draft PR and tag

[sahith-narahari]

cosmos#5878 @aaronc can you take a lot at this, I'm stuck at creating public from secp256r1.
Also which random generator can I use for creating sign bytes

[aaronc]

@sahith-narahari where do you need a random generator?

[robert-zaremba]

Is this task still on? we are doing it in SDK