passage backend; add support based on pass backend

[jenic]

Passage is an Age-backed password manager written by Filippo Valsorda and based on password-store by Jason Donenfeld. Adding support for passage was simply a matter of tweaking the existing support already available under pass for age files instead of GPG.

I've confirmed this works on my system (Arch Linux) by pointing aws-vault's go.mod to my local keyring project and recompiling it. Executing with the new backend, as listed below, properly invokes age and lists my S3 buckets.

./aws-vault-linux-amd64 --backend=passage exec personal -- aws s3 ls

I apologize I did not include a passage_test.go, I started to do so but I am busy with other things at the moment.