Consider adding a mechanism for copying `JsonSerializerOptions` settings to the `JsonOptions` classes

### Background and motivation

Today when I am working with Asp.Net and maybe other libraries/classes that work use JsonSerialization and I want to make sure, that the same options are applied everywhere I have to go through a lot of code.

For example here is how this look like in my AspNet project:

```cs
var builder = WebApplication.CreateBuilder(args);

builder.Services.ConfigureHttpJsonOptions(options =>
{
options.SerializerOptions.PropertyNameCaseInsensitive = true;
options.SerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
options.SerializerOptions.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
options.SerializerOptions.Converters = {
new JsonStringEnumConverter(JsonNamingPolicy.CamelCase),
new MyCustomConverter()
}
options.SerializerOptions.NumberHandling = JsonNumberHandling.AllowReadingFromString;
options.SerializerOptions.UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip;
});

builder.Services
.AddControllers()
.AddJsonOptions(options =>
{
options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
options.JsonSerializerOptions.DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
options.JsonSerializerOptions.Converters = {
new JsonStringEnumConverter(JsonNamingPolicy.CamelCase),
new MyCustomConverter()
}
options.JsonSerializerOptions.NumberHandling = JsonNumberHandling.AllowReadingFromString;
options.JsonSerializerOptions.UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip;
});

builder.Service.AddSingleton(sp =>
{
var serializerOptions = new JsonSerializerOptions
{
PropertyNameCaseInsensitive = true;
PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
Converters = {
new JsonStringEnumConverter(JsonNamingPolicy.CamelCase),
new MyCustomConverter()
}
NumberHandling = JsonNumberHandling.AllowReadingFromString;
UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip;
};

return new CosmosClientBuilder(cosmosDbEndpoint, credential).WithCustomSerializer(
new CosmosSystemTextJsonSerializer(serializerOptions)
).Build();
});
```

As you can see, this can be very cumbersome and it is error-prone. If one changes, I need to make sure, that all of them change, so they stay the same everywhere. Even if you just have the two AspNet Configurations.

### API Proposal

I suggest to add a Merge Method to JsonSerializerOptions that mutates the options.

```csharp
namespace System.Text.Json;

public sealed partial class JsonSerializerOptions
{
public void Merge(JsonSerializerOptions optionsToMerge);
}
```

### API Usage

looking at the example above, I would than be able to do:
```cs
var builder = WebApplication.CreateBuilder(args);
var serializerOptions = new JsonSerializerOptions
{
PropertyNameCaseInsensitive = true;
PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
DictionaryKeyPolicy = JsonNamingPolicy.CamelCase;
Converters = {
new JsonStringEnumConverter(JsonNamingPolicy.CamelCase),
new MyCustomConverter()
}
NumberHandling = JsonNumberHandling.AllowReadingFromString;
UnmappedMemberHandling = JsonUnmappedMemberHandling.Skip;
};

builder.Services.ConfigureHttpJsonOptions(options =>
{
options.SerializerOptions.Merge(serializerOptions);
});

builder.Services
.AddControllers()
.AddJsonOptions(options =>
{
options.JsonSerializerOptions.Merge(serializerOptions);
});

builder.Service.AddSingleton(
sp => new CosmosClientBuilder(cosmosDbEndpoint, credential)
.WithCustomSerializer(new CosmosSystemTextJsonSerializer(serializerOptions))
.Build());
```

### Alternative Designs

Alternatively names for Merge could be `SetFromProperties`, `Update`, I don't care much about the name, it is only important, that it mutates the options, because in the AspNet Configuration Apis, the options are not setable.

### Risks

I don't see any Risk with this, but there are others here, that have much better understanding of this all.