Allow setting HTTP headers

[Lucretius]

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

provider "vault" {
  address = "https://testvault.123"

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

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

I have run the acceptance tests locally and also ran this against a Vault cluster in the cloud that requires specific headers in order for requests to forward to the cluster properly.

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Relates OR Closes #0000

Release note for CHANGELOG:

Added `headers` provider configuration setting to allow setting HTTP headers for all requests to the Vault server.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'
PASS
ok      github.com/terraform-providers/terraform-provider-vault/vault   94.623s

[tyrannosaurus-becks]

This looks great so far! Just a couple minor things needed.

[tyrannosaurus-becks]

Would it be possible to get test coverage of the name and value fields? It would be cool to see what the Terraform config looks like when they're populated.

[Lucretius]

I copied the basic token name test, and just added a custom provider configuration which included a header. It doesn't test any header related functionality (which would depend on individual use cases), only that the headers don't cause errors when Vault is doing things (in the case of this test, token lookup)

[tyrannosaurus-becks]

Thank you!