Add support for lastpass backend

Hi 👋 thanks for the library and for the work on aws-vault,

I'm not sure if this is even possible... reading the source it seems like it is, but I am no Go expert. I would like to implement a [lastpass](https://lastpass.com/) backend for this library. Is that something you would be amenable to receiving?

The issue is about adding a LastPass backend to the keyring library. Start by examining existing backends in the codebase to understand the interface. Research the LastPass API or CLI for credential storage. Determine how to integrate it securely. The work involves implementing a new backend struct and methods, likely in a new file within the backend directory.

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