Allow setting additional HTTP headers

[Mongey]

This PR adds the ability to set HTTP headers for Vault API requests made by the Terraform Vault provider. A sample configuration:

provider "consul" {
  address = "https://consul"

  headers {
    name = "header1"
    value = "valueofheader1"
  }

  headers {
    name = "X-Special-Header"
    value = "specialheadervalue"
  }
}

[remilapeyre]

Thanks for your contribution @Mongey!