Avoid using accidental public API

Motivation:

NIOs underscored Base64 module accidentally leaked some public API as
extensions on `String`. We shouldn't use these.

Modifications:

- Vend base64 encoding
- Use it instead of NIOs API

Result:

Avoid using accidentally public API

NOTICES.txt

@@ -0,0 +1,25 @@
+                          The gRPC Swift Project
+                          ======================
+
+Copyright 2021 The gRPC Swift Project
+
+The gRPC Swift project licenses this file to you under the Apache License,
+version 2.0 (the "License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at:
+
+  https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+License for the specific language governing permissions and limitations
+under the License.
+
+-------------------------------------------------------------------------------
+
+This product uses derivations of swift-extras/swift-extras-base64 'Base64.swift'.
+
+  * LICENSE (Apache License 2.0):
+    * https://github.com/swift-extras/swift-extras-base64/blob/main/LICENSE
+  * HOMEPAGE:
+    * https://github.com/swift-extras/swift-extras-base64

Sources/GRPCNIOTransportCore/GRPCStreamStateMachine.swift

@@ -1866,7 +1866,7 @@ extension Metadata {
     for header in headers {
       if header.name.hasSuffix("-bin") {
         do {
-          let decodedBinary = try header.value.base64Decoded()
+          let decodedBinary = try Base64.decode(string: header.value)
           metadata.addBinary(decodedBinary, forKey: header.name)
         } catch {
           metadata.addString(header.value, forKey: header.name)