feat: Add retry on rate limit errors

[unexge]

While working on #1008 I was using an AWS account with a lower request limit, and I was getting throttling exceptions frequently similar to what's reported on #887. This PR adds a new environment variable GOOSE_AGENT_RETRY_MULTIPLIER_MS to enable retries on ProviderError::RateLimitExceeded errors from Providers in TruncateAgent. It uses a very simple retry with exponential backoff.

For example, I tried to add a MetricAgent to goose, and after some iterations I got a rate limit error:

then I enabled retry by changing my config:

and this time after rate limits, goose agent retries the request and got success on the second retry:

Of course if the limit is too low it will probably do lots of unsuccessful retries, but I feel like this could be helpful in some cases.