Make signature verification non-fatal for Drive operations

[cnighswonger]

Summary

• After enabling 2FA on a Proton account, address keys are rotated. Existing Drive data (folders, files, shares) was signed with the old keys, causing VerifyDetached and Decrypt-with-verification calls to fail with "No matching signature"
• This patch makes signature verification best-effort across all Drive-related operations — if verification fails, it falls back to decryption without signature checking
• Encryption is not weakened — data is still decrypted with the correct node/share keys. Only the metadata integrity check (that data was signed by a specific address key) is relaxed

Affected functions

Function	Change
Link.GetName	Fall back to Decrypt without verification keyring
Link.GetKeyRing	VerifyDetached result ignored
Link.GetHashKey	Fall back to Decrypt without verification
Link.GetSessionKey	VerifyDetached result ignored
Share.GetKeyRing	VerifyDetached result ignored
RevisionMetadata.GetDecXAttrString	Fall back to Decrypt without verification
Revision.GetDecXAttrString	Fall back to Decrypt without verification

Motivation

This fixes a class of hard failures that affect real users:

1. Enabling 2FA rotates address keys → all existing signatures become invalid
2. Email alias changes (@proton.me vs @protonmail.com) can cause signature email mismatches

These are legitimate account operations that should not make existing Drive data inaccessible. The Proton web client handles this gracefully; rclone should too.

Companion PR: rclone/Proton-API-Bridge#6

Fixes rclone/rclone#8003, rclone/rclone#7344

Test plan

• Verified fix resolves the issue on a live Proton Drive account after enabling 2FA
• Listing folders, listing files, file metadata decryption, and file download all succeed
• Combined with the Proton-API-Bridge keyring fallback fix, the full rclone protondrive backend works end-to-end

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