openpgp: enable signing using a subkey that does not have the primary key private material

[djgilcrease]

This addresses issue golang/go#23226 which allows users to follow the best practice of keeping their primary key offline.
https://riseup.net/en/security/message-security/openpgp/best-practices#have-a-separate-subkey-for-signing

closes: golang/go#13605
closes: golang/go#23226
closes: golang/go#9312

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This PR is being closed because golang.org/cl/283234 has been abandoned.

The x/crypto/openpgp package is now frozen, so it won't accept any more non-security changes. Thank you for opening a PR and sorry for the significant lag. You might want to consider submitting this change to one of the community forks of this package instead. See https://golang.org/issues/44226 for more information about this decision.