Add support for custom headers so that we can audit them

[Constantin07]

It makes sense to be able to send custom HTTP headers to Vault so that they can be included in vault audit logs. These headers can include additional metadata to be tracked in audit logs.

Usage scenario: application running in docker container when requesting credentials from vault ca n post, for instance two custom headers like 'X-Application-Name: app-name' and 'X-Application-Version: 1.0.0' which will appear in vault audit logs.

Via API (curl) this can be done as:

curl -sk -H "X-Vault-Token: <token>" -H "X-Application-Name: app-test" -X GET https://***.****.***:8200/v1/secret/custom 

Then in audit logs we can capture something like:

    "request": {
      "id": "ef5e02ca-b9ae-8846-6835-1c91168890c3",
      "operation": "read",
      "client_token": "hmac-sha256:911623f1a0e5dfd31368c107b62782dc1375dd28a20f01068df69a1187f4891f",
      "client_token_accessor": "hmac-sha256:c9336f259dc9201577f7fb8efebcdfa74497f4a66afd108df888b293e73f605b",
      "path": "secret/custom",
      "data": null,
      "remote_address": "172.19.0.28",
      "wrap_ttl": 0,
      "headers": {
        "x-application-name": [
          "app-test"
        ]
      }

This will allow to track from which application the request came from. At the moment the audit logs include source IP which doesn't help.

[DaniGuardiola]

This is totally possible and probably even easy to implement. I will try to find time to implement this after finishing with my refactor pull request

[aviadhahami]

Since #78 is stale, I'm closing this issue as well. That being said - will keep in mind the requirement of allowing headers access