Changing PostgresStreamSubscriptionOptions.Stream to be mutable

[matt-lethargic]

Changing PostgresStreamSubscriptionOptions.Stream to be mutable, this now is more in-line with how the documentation outlines it's usage:

builder.Services.AddSubscription<PostgresStreamSubscription, PostgresStreamSubscriptionOptions>(
    "StreamSubscription",
    builder => builder
        .Configure(x => x.StreamName = "my-stream")
        .AddEventHandler<StreamSubscriptionHander>()
);

but the document should now be:

builder.Services.AddSubscription<PostgresStreamSubscription, PostgresStreamSubscriptionOptions>(
    "StreamSubscription",
    builder => builder
        .Configure(x => x.StreamName = new StreamName("my-stream"))
        .AddEventHandler<StreamSubscriptionHander>()
);

and is now closer to how StreamPersistentSubscriptionOptions is created.

(documentation site will need a PR if this is accepted)

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