Feature: configurable passphrase prompt

[kshpytsya]

I am am using age with SOPS. Age key is passphrase encrypted with age itself (age-keygen | age -p -a > encrypted_age_key). SOPS is configured to use this via env variable (export SOPS_AGE_KEY_CMD="age -d .../encrypted_age_key"). This works and I get a prompt for passphrase when SOPS needs the key. However, the prompt is a pretty generic Enter passphrase: which is not that good when dealing with sensitive material. My first thought was to add an echo to SOPS_AGE_HEY_CMD but no amount of sh -c ... magic made SOPS accept the result. My request is to add an option to age to specify passphrase prompt.

[kshpytsya]

P.S. while it turns out that a SOPS was fixed to read encrypted age identity files (https://github.com/getsops/sops/pull/1400) I believe this feature could be of general use.