feat: add support for AWS bedrock hosted models

[dbkegley]

I decided to take a stab at #58 over the weekend. Creating this as draft for now because there's a bit of cleanup to be done still. Is this something that would be accepted if I spent some time cleaning everything up and adding tests?

I'm no rust developer so please don't assume anything you see in this PR was on purpose.

Implementation Notes

Adds a new awsbedrock api type. The only supported option is default_model.
Uses the rust aws-sdk to load the user's default credentials for their current environment. I considered adding a aws-profile option to the api config to allow overriding the aws credentials being used but decided to omit it for now.

The aws client is async so I had to add a direct dependency on tokio. I don't think this is a huge deal since the reqwest blocking client also uses tokio.

The errors returned from the aws-sdk are difficult to debug so that needs to be improved (maybe with a --debug flag?)

Adds a new ApiClient trait with ReqwestClient and AwsClient implementations.
Adds a new ApiError type.