Add explicit access-level modifier support for imports in code generator (#2010)

## Motivation
This is a follow-up of grpc/grpc-swift#2003.
As of 5.9, Swift supports access-level modifiers on imports. In an
upcoming Swift 6.x release, the default modifier will become `internal`.

## Modifications
This PR adds explicit access-level modifiers to the generated code and a
few other modules that use this generated code.

## Result
Explicit access level imports present in more places.

Author: gjcairo

Sources/InteroperabilityTests/Generated/empty_service.grpc.swift

@@ -21,8 +21,8 @@
 // For information on using the generated types, please see the documentation:
 //   https://github.com/grpc/grpc-swift
 
-import GRPCCore
-import GRPCProtobuf
+public import GRPCCore
+internal import GRPCProtobuf
 
 public enum Grpc_Testing_EmptyService {
     public static let descriptor = ServiceDescriptor.grpc_testing_EmptyService

Sources/InteroperabilityTests/Generated/test.grpc.swift

@@ -24,8 +24,8 @@
 // For information on using the generated types, please see the documentation:
 //   https://github.com/grpc/grpc-swift
 
-import GRPCCore
-import GRPCProtobuf
+public import GRPCCore
+internal import GRPCProtobuf
 
 public enum Grpc_Testing_ReconnectService {
     public static let descriptor = ServiceDescriptor.grpc_testing_ReconnectService

Sources/Services/Health/Generated/health.grpc.swift

@@ -24,8 +24,8 @@
 // For information on using the generated types, please see the documentation:
 //   https://github.com/grpc/grpc-swift
 
-import GRPCCore
-import GRPCProtobuf
+package import GRPCCore
+internal import GRPCProtobuf
 
 package enum Grpc_Health_V1_Health {
     package static let descriptor = ServiceDescriptor.grpc_health_v1_Health

Sources/Services/Health/Health.swift

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import GRPCCore
+public import GRPCCore
 
 /// ``Health`` is gRPC’s mechanism for checking whether a server is able to handle RPCs. Its semantics are documented in
 /// https://github.com/grpc/grpc/blob/master/doc/health-checking.md.

Sources/Services/Health/HealthService.swift

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import GRPCCore
+internal import GRPCCore
 
 @available(macOS 15.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 internal struct HealthService: Grpc_Health_V1_HealthServiceProtocol {