pinentry.el: Support external passphrase cache

pinentry.el: Support external passphrase cache

* lisp/net/pinentry.el (pinentry-use-secrets): New user option.
(pinentry--allow-external-password-cache): New local variable.
(pinentry--key-info): New local variable.
(secrets-enabled, secrets-search-items, secrets-get-secret):
Declare.
(pinentry--send-passphrase): New function, split from
`pinentry--process-filter'.
(pinentry--process-filter): Use secrets.el to retrieve passphrase
from login keyring.