Generic Auth needs configurable headers

[sriedel]

I'm trying to setup an AWS IAM authentication with the generic auth module.

However, for this specific authentication method the X-Vault-AWS-IAM-Server-ID header set (q.v. https://developer.hashicorp.com/vault/api-docs/auth/aws#iam_request_headers). So a merge from the request parameters is needed, similar to what is already happening with the body params.

The change seems simple, so I'll see if I can give a PR a shot.