Allow application/x-pem-file content type

[erpel]

This refers to PEM encoded files often used for certificates and related
data.

Example source for this content type: ietf-wg-acme/acme#120

[hashicorp-cla]

All committers have signed the CLA.

[lucymhdavies]

On a related note, recognising application/pem-certificate-chain would be useful.
Perhaps, rather than hard-coding specific content types we expect, we should add a parameter that would allow the user to specify what content type they are expecting.

In my case:

data "http" "ca" {
  url = "https://vault.fancycorp.io/v1/pki/root/ca/pem"
  expected_content_types = [
    "application/pem-certificate-chain"
  ]
}

@erpel's use-case would likely be similar.

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