[mcp-account-manager] Implement get_additional_info to provide display names

This provides both the account and provider display name to clients.
[mcp-account-manager] Use account name for DisplayName again

The account name can change, but provider name cannot. DisplayName is
able to signal changes, while storage info is not.
Merge pull request #10 from special/additional_info

[mcp-account-manager] Implement get_additional_info to provide display names