IAM authentication does not allow header_value and role to be configured

[pieter-lautus]

HashiCorp's docs recommend that the IAM auth method be configured to use a nonce (X-Vault-AWS-IAM-Server-ID).

In addition, when the role name in Vault is different from the IAM role, the role name must be specified as well.

This plugin does not allow either values to be configured.

To be clear: as per HashiCorp's docs, a client that authenticates using the IAM method on the CLI would do something like the following:

vault login -method=aws header_value=vault.example.com role=dev-role-iam

This plugin needs to allow for the equivalent of the header_value and role above to be specified.