Validate keypair app and command

[psteckler]

New validate_keypair app, and command advanced validate-keypair.

As with generate-keypair, the app and command use CODA_PRIVKEY_PASS, if set, and prompt for a password otherwise, and both take the privkey-path flag.

Validation consists of signing a dummy transaction and verifying the signature.

TODO: add a test of the keys using the VRF machinery (as commented in the code).

Update: won't add VRF bits, see comment below.

Example session:

$ coda advanced validate-keypair -privkey-path ~/keys/flimflam
Enter password: 
Again to confirm: 
Verified a transaction using specified keypair

[lk86]

Looks good from product + infra view, but still waiting on the VRF changes before we ship it

[mrmr1993]

The VRF is effectively a hash function built out of a signature. The only test you can do is to re-evaluate it and check that they're equal, which doesn't seem to give us anything of value. eval_and_check_public_key is for snarks, so the additional checks are not relevant for normal use.

[bkase]

In other words, you're saying there is no possible universe that a key could properly sign transactions but not evaluate VRFs? I guess if this is the case, then we don't need that extra check.