Add support for SSE-S3 server side encryption with vault (#6192)

Add support for sse-s3 encryption with vault as KMS.

Also refactoring code to make use of headers and functions defined in
crypto package and clean up duplicated code.

vendor/github.com/hashicorp/go-rootcerts/README.md

@@ -0,0 +1,43 @@
+# rootcerts
+
+Functions for loading root certificates for TLS connections.
+
+-----
+
+Go's standard library `crypto/tls` provides a common mechanism for configuring
+TLS connections in `tls.Config`. The `RootCAs` field on this struct is a pool
+of certificates for the client to use as a trust store when verifying server
+certificates.
+
+This library contains utility functions for loading certificates destined for
+that field, as well as one other important thing:
+
+When the `RootCAs` field is `nil`, the standard library attempts to load the
+host's root CA set.  This behavior is OS-specific, and the Darwin
+implementation contains [a bug that prevents trusted certificates from the
+System and Login keychains from being loaded][1]. This library contains
+Darwin-specific behavior that works around that bug.
+
+[1]: https://github.com/golang/go/issues/14514
+
+## Example Usage
+
+Here's a snippet demonstrating how this library is meant to be used:
+
+```go
+func httpClient() (*http.Client, error)
+	tlsConfig := &tls.Config{}
+	err := rootcerts.ConfigureTLS(tlsConfig, &rootcerts.Config{
+		CAFile: os.Getenv("MYAPP_CAFILE"),
+		CAPath: os.Getenv("MYAPP_CAPATH"),
+	})
+	if err != nil {
+		return nil, err
+	}
+	c := cleanhttp.DefaultClient()
+	t := cleanhttp.DefaultTransport()
+	t.TLSClientConfig = tlsConfig
+	c.Transport = t
+	return c, nil
+}
+```