cmd: Support client secret encryption at stdout

[sawadashota]

Is your feature request related to a problem? Please describe.

When creating clients via command, hydra output client id and secret to stdout.
But in some cases, this is inconvenience.

1. When a person who executes hydra clients create might not be client application developer, he/she should not get to know client secret.
2. When logging stdout for docker container, I'm nervous to execute command hydra clients create not to log.

Describe the solution you'd like

Add option of client secret encryption to hydra clients create using PGP key.

A good sample is here.
Terraform AWS Provider provides option to encrypt IAM User's password.
https://www.terraform.io/docs/providers/aws/r/iam_user_login_profile.html

To apply this, command line interface will be following

• hydra clients create --pgp-key=<base64 encoded PGP encryption key> ...
• hydra clients create --pgp-key-url=<PGP encryption key URL> ...
• hydra clients create --keybase=<Keybase username> ...

Then stdout will be like following

OAuth 2.0 Client ID: 15af00e5-6485-4f4b-bca7-713fc4868ff0
OAuth 2.0 Client Secret: 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

To decrypt this, private key holder executes base64 decode and PGP decrypt.
Following is an example for keybase.

$ echo ${client_secret} | base64 --decode | keybase pgp decrypt
# raw password here