Add ability to add private keys to trust store

[pmiles]

@kdubb - I have a use case where I need the following in my trust store:

CA cert
self signed cert
self signed cert's private key (in PEM format)

When I attempt to the the following TF, I get the error Error: chain 1, certificate 0: expected CERTIFICATE but found "EC PRIVATE KEY"

resource "tls_private_key" "es_key" {
  algorithm = "ECDSA"
}

resource "jks_trust_store" "ca" {
  certificates = [
    tls_private_key.es_key.private_key_pem,
  ]
  password = "none"
}

The small PR #2 fixes this.