fix importing softsign secp256k1 key type

[mkaczanowski]

Description

This change along with updated (prior this PR) version of tendermint-rs adds support for softsign import of secp256k1 key type. Tested via:

$ cat test_priv_validator_key.json
{
  "address": "0799B6E1F13F1ECC7E891765F499A73665F42866",
  "pub_key": {
    "type": "tendermint/PubKeySecp256k1",
    "value": "A2hdb2LlTQ+sxqpknkA2v0RgQ6a6Q4t5OwhebIqSr8qu"
  },
  "priv_key": {
    "type": "tendermint/PrivKeySecp256k1",
    "value": "aVmw0npSBEqPU4v6qEZ9o3z5nCpHjicwFDdIBRUbSrM="
  }
}

$ ./target/debug/tmkms softsign import test_priv_validator_key.json out.key
2023-10-16T21:25:06.303245Z  INFO tmkms::commands::softsign::import: Imported Secp256k1 private key to out.key

$ cat out.key 
aVmw0npSBEqPU4v6qEZ9o3z5nCpHjicwFDdIBRUbSrM=

Fixes

#761

[tony-iqlusion]

Thanks!