Add in-process test cluster

[ReubenBond]

This PR adds an in-process test cluster, InProcessTestCluster. The advantages of this over the existing TestCluster are:

• Silos & clients can be configured using simple delegates (rather than 'configurator' types). This makes it much easier to configure shared dependencies across silos & clients
• Silos expose their IServiceProvider, so dependencies can be retrieved and called as part of your test (eg, for fault injection, state inspection)
• The cluster uses a new in-memory clustering & directory provider, so tests which start & stop silos do not need to worry about stopping the 'Primary' silo.

Here's an example:

// Setup
var builder = new InProcessTestClusterBuilder(initialSilosCount: 4);
builder.ConfigureClient(c => c.AddMemoryStreams("foo"));
builder.ConfigureSilo((siloOptions, siloBuilder) => siloBuilder.AddMemoryStreams("foo"));
await using var cluster = builder.Build();
await cluster.DeployAsync();

// Act
var value = await cluster.Client.GetGrain<IEchoGrain>(Guid.NewGuid()).Echo("test");

// Assert
Assert.Equal("test", value);

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