add SSE-C support for HEAD, GET, PUT (#4894)

This change adds server-side-encryption support for HEAD, GET and PUT
operations. This PR only addresses single-part PUTs and GETs without
HTTP ranges.

Further this change adds the concept of reserved object metadata which is required
to make encrypted objects tamper-proof and provide API compatibility to AWS S3.
This PR adds the following reserved metadata entries:
- X-Minio-Internal-Server-Side-Encryption-Iv          ('guarantees' tamper-proof property)
- X-Minio-Internal-Server-Side-Encryption-Kdf         (makes Key-MAC computation negotiable in future)
- X-Minio-Internal-Server-Side-Encryption-Key-Mac     (provides AWS S3 API compatibility)

The prefix `X-Minio_Internal` specifies an internal metadata entry which must not
send to clients. All client requests containing a metadata key starting with `X-Minio-Internal`
must also rejected. This is implemented by a generic-handler.

This PR implements SSE-C separated from client-side-encryption (CSE). This cannot decrypt
server-side-encrypted objects on the client-side. However, clients can encrypted the same object
with CSE and SSE-C.

This PR does not address:
 - SSE-C Copy and Copy part
 - SSE-C GET with HTTP ranges
 - SSE-C multipart PUT
 - SSE-C Gateway

Each point must be addressed in a separate PR.

Added to vendor dir:
 - x/crypto/chacha20poly1305
 - x/crypto/poly1305
 - github.com/minio/sio

cmd/generic-handlers_test.go

@@ -144,3 +144,38 @@ func TestIsHTTPHeaderSizeTooLarge(t *testing.T) {
 		}
 	}
 }
+
+var containsReservedMetadataTests = []struct {
+	header     http.Header
+	shouldFail bool
+}{
+	{
+		header: http.Header{"X-Minio-Key": []string{"value"}},
+	},
+	{
+		header:     http.Header{ServerSideEncryptionIV: []string{"iv"}},
+		shouldFail: true,
+	},
+	{
+		header:     http.Header{ServerSideEncryptionKDF: []string{SSEKeyDerivationHmacSha256}},
+		shouldFail: true,
+	},
+	{
+		header:     http.Header{ServerSideEncryptionKeyMAC: []string{"mac"}},
+		shouldFail: true,
+	},
+	{
+		header:     http.Header{ReservedMetadataPrefix + "Key": []string{"value"}},
+		shouldFail: true,
+	},
+}
+
+func TestContainsReservedMetadata(t *testing.T) {
+	for i, test := range containsReservedMetadataTests {
+		if contains := containsReservedMetadata(test.header); contains && !test.shouldFail {
+			t.Errorf("Test %d: contains reserved header but should not fail", i)
+		} else if !contains && test.shouldFail {
+			t.Errorf("Test %d: does not contain reserved header but failed", i)
+		}
+	}
+}