Use FoundationEssentials where possible (#17)

Motivation:

FoundationEssentials only includes ... the essentials. We should use it
where available.

Modifications:

- Replace Foundation imports with FoundationEssentials import

Result:

Smaller dependency set

Author: glbrntt

Sources/GRPCInteropTests/InteroperabilityTestCases.swift

@@ -17,7 +17,11 @@
 internal import GRPCCore
 private import SwiftProtobuf
 
+#if canImport(FoundationEssentials)
+private import struct FoundationEssentials.Data
+#else
 private import struct Foundation.Data
+#endif
 
 /// This test verifies that implementations support zero-size messages. Ideally, client
 /// implementations would verify that the request and response were zero bytes serialized, but

Sources/GRPCInteropTests/TestService.swift

@@ -14,10 +14,15 @@
  * limitations under the License.
  */
 
-private import Foundation
 public import GRPCCore
 private import SwiftProtobuf
 
+#if canImport(FoundationEssentials)
+private import struct FoundationEssentials.Data
+#else
+private import struct Foundation.Data
+#endif
+
 public struct TestService: Grpc_Testing_TestService.ServiceProtocol {
   public init() {}