Feature Request: Support bech32 private key

[Thoralf-M]

Right now the config only accepts a base64 encoded private key, but in the CLI and wallet, the bech32 format is preferred, would be good to also support that here
Would also make the setup easier, for example this step here would not be needed anymore

gas-station/README.md

Line 140 in 50eda00

3. Convert the bech32 key to base64: `./utils/gas-station-tool.sh convert-key --key iotaprivatkey...`