Add secp256r1 Signing Keys

[aaronc]

Summary

Add support for singing tranactions with the secp256r1/NIST P-256 algorithm (used in macOS/iOS/watchOS & Android secure enclaves). Already discussed in #4789 and #7074 but there was no standalone issue.

Problem Definition

Supporting the signing algorithm for the secure enclave in macOS/iOS/watchOS and Android Hardware-backed Keystore will allow these devices to basically function as hardware wallets. There has been speculation that the secp256r1/NIST P-256 algorithm was backdoored by the NSA, but this is unproven and users should be able to choose their desired level of security. For many users, having a smartphone-backed key for low security operations (as enabled by the other key management modules #7074) may be the best options.

Proposal

• add a secp256r1/NIST P-256 PubKey implementation with addresses based on ADR 028
• add support to the ante handler with a proper gas param
• DON'T add support in the keyring because this is for mobile/desktop clients ONLY

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[zmanian]

Strong support.

[clevinson]

@aaronc Is it possible that this gets put in a point release after the initial v0.42 is cut?

[aaronc]

@aaronc Is it possible that this gets put in a point release after the initial v0.42 is cut?

Sure, but it would probably need to be 0.43