Restructure in-process transport (#2071)

Motivation:

To create a connected pair of `ServerTransport` and `ClientTransport`,
users need to call `InProcessTransport.makePair(serviceConfig:)` which
returns a tuple. The spelling of this API can be more intuitive and
simple.

Modifications:

- Restructure `InProcessTransport` to include two properties: `server`
(the `ServerTransport`) and `client` (the `ClientTransport`), and an
`init(serviceConfig:)` that pairs these properties.
- Add missing GRPCInProcessTransport dependency to GRPCCore test target.

Result:

The spelling of the `InProcessTransport` API will be better.

Author: clintonpi

Sources/GRPCCore/GRPCClient.swift

@@ -65,11 +65,10 @@ private import Synchronization
 /// )
 ///
 /// // Finally create a transport and instantiate the client, adding an interceptor.
-/// let inProcessServerTransport = InProcessServerTransport()
-/// let inProcessClientTransport = InProcessClientTransport(serverTransport: inProcessServerTransport)
+/// let inProcessTransport = InProcessTransport()
 ///
 /// let client = GRPCClient(
-///   transport: inProcessClientTransport,
+///   transport: inProcessTransport.client,
 ///   interceptors: [StatsRecordingClientInterceptor()],
 ///   configuration: configuration
 /// )

Sources/GRPCCore/GRPCServer.swift

@@ -35,7 +35,7 @@ private import Synchronization
 ///
 /// ```swift
 /// // Create and an in-process transport.
-/// let inProcessTransport = InProcessServerTransport()
+/// let inProcessTransport = InProcessTransport()
 ///
 /// // Create the 'Greeter' and 'Echo' services.
 /// let greeter = GreeterService()
@@ -46,7 +46,7 @@ private import Synchronization
 ///
 /// // Finally create the server.
 /// let server = GRPCServer(
-///   transport: inProcessTransport,
+///   transport: inProcessTransport.server,
 ///   services: [greeter, echo],
 ///   interceptors: [statsRecorder]
 /// )