Add share-link update command

[AndreyVMarkelov]

Summary

• Add dbxcli share link update <url> to modify shared link settings
• Flags: --expires <RFC3339>, --remove-expiration, --allow-download
• Validates mutual exclusion of --expires and --remove-expiration
• Requires at least one setting flag (prevents no-op API calls)
• Silent on success; verbose mode prints status to stderr
• Extends sharedLinkClient interface with ModifySharedLinkSettings

Test plan

• Requires exactly one URL argument (nil and multi-arg rejected)
• Rejects empty URL with clear error
• Requires at least one setting flag
• Rejects invalid RFC3339 timestamp for --expires
• Rejects --expires and --remove-expiration together
• Sets expiration correctly on API arg
• Removes expiration correctly (RemoveExpiration = true)
• Sets allow-download correctly
• Verbose success writes status to stderr
• Success produces no stdout output
• API errors returned unchanged
• Command registered under share link
• golangci-lint clean, all tests pass

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