Don't override existing authentication headers

I'd like to be able to run the extension to get the caching functionality, but do my own authentication. It seems like the auth header [is added unconditionally](https://github.com/hashicorp/vault-lambda-extension/blob/14b548bd4b6e1aef4f6e45a410949458320bd0a4/internal/proxy/proxy.go#L140-L141) - could this be changed to only add the header if it is not present in the original request?

My use case is a Lambda function that post-processes some PKI certificates produced by Vault. So the user will do their own authentication, and that should be passed through unaltered to Vault. Having the extension's cache functionality would be useful to reduce duplicate credentials created if the user refreshes the page.