Implement support for TPM-backed signing keys (#953)

[neolynx]

Replaces #955

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS

[neolynx]

rebased on master, build error:

pgp/internal.go:292:41: cannot use g.signer (variable of type *"github.com/ProtonMail/go-crypto/openpgp".Entity) as *"golang.org/x/crypto/openpgp".Entity value in argument to tpmk.OpenPGPDetachSign