Allow serializer settings to be configured

[benfoster]

There are two factors that influence the way in which events are serialized to CosmosDB.

1. The settings provided when intializing the Cosmos client, for example:

static CosmosClient CreateCosmosClient(string accountEndpoint)
    => new(
        accountEndpoint: accountEndpoint,
        tokenCredential: new DefaultAzureCredential(),
        new CosmosClientOptions
        {
            ApplicationName = "psp-payment-processor",
            Serializer = new CosmosJsonSerializer(SerializerSettings.Default)
        }
    );

2. The settings used when serializing the event payload (from CosmosEvent.cs):

    public static CosmosEvent FromEventData(string streamId, long eventNumber, EventData @event, JsonSerializer serializer)
        => new()
        {
            Id = GenerateId(eventNumber, streamId),
            EventId = @event.Id,
            StreamId = streamId,
            EventType = @event.EventType,
            Data = JObject.FromObject(@event.Data, serializer),
            EventNumber = eventNumber,
            Metadata = @event.Metadata
        };

The latter was hardcoded to SerializerSettings.Default so regardless of the settings configured on the Cosmos client, they wouldn't have any impact on the serialization of the actual event payload.

This PR makes it configurable.