Add convert cli command for converting tendermint consensus key from namada wallet keys

[Jeongseup]

Feature

Add convert cli command in namadaw

Purpose

Sometimes, Validators want to change each own consensus-key for security management and it's possible to use change-consensus-key command in namada client.

So, I used this command for changing my consensus key in the namada testnet. and then, I faced with a problem like things. That was there wasn't how to generate priv_validator_key.json file in the docs, after creating a new consensus-key.

For that reason, I make this issue to implement converting(or generating) priv_validator_key.json from with created a consensus key(actually in the wallet.toml) and will request PR for about this issue

Expectation

After changing a consesus key, the key is located in your wallet.toml. So, we need to convert key from wallet.toml

It'll be like namadaw convert --alias <key-name>