Add Bedrock Cloud Model provider

[5herlocked]

Closes #16544

Release Notes:

• Added support for Bedrock as an FM provider
• Added rudimentary support for AWS credential management to Zed

[5herlocked]

This still needs to be updated to the new method of managing models, going to spend the next couple of weeks migrating to it.

[5herlocked]

Zed maintainers, would appreciate some guidance on how the ZedPro things are setup and if it makes sense to pass the Bedrock models into it.

On a cursory reading of the feature, i don't believe it does -- however, i am open to working with the bedrock service team to make it a reality.

[rgbkrk]

I don't think you need to pass through the bedrock models through the collab service, as Zed would need to run their own bedrock setup. I'd just do the API key / AWS credentials route.

[SomeoneToIgnore]

Unfortunately, our CLA bot is broken, so I have to substitute for it for a while:

We require contributors to sign our Contributor License Agreement, and we don't have @5herlocked on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[5herlocked]

Will get it done post a legal review of the docs (just want to make sure AWS doesn't have an issue with it)