agent: add disable_idle_connections configurable

[jasonodonnell]

A commit to go-retryablehttp changed the default behavior of closing every successful request to instead keeping the connection open. This was done to allow users of the package to utilize TCP keep alives. This resulted in a large number of connections being left open for 90 seconds by Vault Agent 1.6.0+.

This PR adds a new configurable, disable_idle_connections, which takes a string list to disable idle connections for various features in Vault Agent (auto-auth, caching and templating).

A small example can be found here:

 "auto_auth" = {
  "method" = {
    "config" = {
      "role" = "app"
    }
    "type" = "kubernetes"
  }

  "sink" = {
    "config" = {
      "path" = "/home/vault/.token"
    }
    "type" = "file"
  }
}

"disable_idle_connections" = ["auto-auth", "templating"]
"exit_after_auth" = false
"pid_file" = "/home/vault/.pid"

"template" = {
  "contents" = "{{ with secret \"secret/hashiconf\" }}{{ .Data | toJSONPretty }}{{ end }}"
  "destination" = "/vault/secrets/kv-secret"
  "error_on_missing_key" = true
}

"vault" = {
  "address" = "https://vault.vault.svc.cluster.local:8200"
}

[swenson]

LGTM