Add capability to generate secret values

[sk3w]

I added a quick feature to automatically generate new secret values and add them to the config. Is this something you'd find useful?

> amber generate KEY1
Your new secret value is KEY1: awUHt_Z9UKmYzDp2HKPz8A==

❯ cat amber.yaml
---
file_format_version: 1
public_key: <redacted>
secrets:
  - name: KEY1
    sha256: dba4b63c27dc8827b465594894f6e5c6901a75799ca7b5c00c1b7740128237a8
    cipher: <redacted>

[snoyberg]

Looks like a great addition, thanks! Can you also include a line in the changelog about the new subcommand?

[snoyberg]

I wonder if we should include a command line flag to control how many bytes of entropy are included here. What do you think?

[sk3w]

I was leaning towards the hardcoded value for simplicity and footgun-avoidance, but I guess there is a potential use-case when secrets of a speicific size are required? (Though the cases I can think of at the moment would also have their own preferred tools for generating & formatting the required values.)

[snoyberg]

Fair enough, we can leave as-is until there's a specific requirement.

[snoyberg]

Thanks!