feat: consider WebAuthn & SSH for instance signing (#7693)

- Currently the options `pubkey` and `twofa` only consider TOTP and GPG keys respectively. Adjust the code to also consider WebAuthn credentials and SSH keys.
- While adding the new unified functions I noticed that certain places also benefited from using these unified functions and took the liberty (where it was either a trivial translation or it was covered under testing) to use the new unified functions.
- Resolves forgejo/forgejo#7658
- Adds unit and integration tests.

Documentation PR: https://codeberg.org/forgejo/docs/pulls/1166

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7693
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/asymkey/asymkey.go

@@ -0,0 +1,29 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+package asymkey
+
+import (
+	"context"
+
+	"forgejo.org/models/db"
+)
+
+// HasAsymKeyByUID returns true if the user has a GPG key or SSH key associated
+// with its account.
+func HasAsymKeyByUID(ctx context.Context, userID int64) (bool, error) {
+	hasGPGKey, err := db.Exist[GPGKey](ctx, FindGPGKeyOptions{
+		OwnerID:        userID,
+		IncludeSubKeys: true,
+	}.ToConds())
+	if err != nil {
+		return false, err
+	}
+	if hasGPGKey {
+		return true, nil
+	}
+
+	return db.Exist[PublicKey](ctx, FindPublicKeyOptions{
+		OwnerID:  userID,
+		KeyTypes: []KeyType{KeyTypeUser},
+	}.ToConds())
+}