feat: add bitwarden secrets manager backend

[Khabi]

Description

Fixes:

Checklist

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Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

Other information

Bitwarden Secrets Manager addition.

The service has a free tier, so its helpful for home users labs.

Their SDK has a dependecy on CGO. I think that may be a sticking point in some projects, so I'm pointing it out ahead of time. That requires build changes to this repository to support it as well.

I see there is a Darwin build option, but I do not have access to validate that it works with the changes I've added.

[Khabi]

If the CGO addition is going to be an issue then I completely understand. I'm not a huge fan of it myself, but I wasn't able to find a bitwarden library that was pure GO.