[age] support named recipients and references

[lttb]

Hey!

currently, the only way to track recipients is through comments.
while this works, if different profiles have different recipient sets, we'd need to duplicate the keys and comments for any shared recipients.

I'd like to suggest an option to define recipients, for example, as a table rather than an array of strings.
this way, recipients can be self-documented and optionally referenced in profiles if needed

to illustrate the idea:

[providers.age]
type = "age"
[providers.age.recipients]
admin     = "key"
alice     = "key"
bob       = "key"
github_ci = "key"

[profiles.staging.providers.age]
recipients_ref = ["alice", "bob"]
type           = "age"

[profiles.production.providers.age]
recipients_ref = ["admin", "github_ci"]
type           = "age"