Mark imported priv_validator keys as exportable

[mdyring]

Fixes #245 for imported priv_validator keys.

[tarcieri]

Perhaps it'd make sense to just add EXPORTABLE_UNDER_WRAP to the DEFAULT_CAPABILITIES.

If we did that, this would need to be updated too:

https://github.com/tendermint/kms/blob/69157e4adddead46b31fed8b032276c25975b32b/src/commands/yubihsm/keys/generate.rs#L76

...but right now the logic there is oddly inverted anyway (silly me), so it would actually be more straightforward I think to remove the EXPORTABLE_UNDER_WRAP capability if the key is explicitly configured to be non-exportable.

[mdyring]

Agree it would be nicer, unfortunately I am not fluent in Rust so my feeble attempt is failing so far:

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
  --> src/commands/yubihsm/keys/mod.rs:20:55

20 | pub const DEFAULT_CAPABILITIES: yubihsm::Capability = yubihsm::Capability::SIGN_EDDSA | yubihsm::Capability::EXPORTABLE_UNDER_WRAP;

[tarcieri]

@mdyring ok, in that case I think this is fine for now and I can try to submit a followup PR to do what I was describing later