C# Plugin Should Allow Generating Partial Classes

### Which packages are impacted by your issue?

@graphql-codegen/c-sharp

### Describe the bug

The C# plugin should be configurable to allow generating C# partial classes. See docs: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/partial-type

This gives C# developers more flexibility to enhance the generated types and it also allows to fix the current shortcoming of the plugin regarding GraphQL union types, which aren't supported.

If a generated class A is marked with the `partial` keyword, the developer can add another file next to the generated one and enhance the class A, e.g. make it inherit from an interface to fix the unsupported union type or add helper methods.

Class generated by the plugin `GeneratedTypes.cs`
```
internal partial class GeneratedTypes
{
internal partial class A
{
public int Idx { get; set; }
}
}
```

Class manually added by the dev `GeneratedTypes.Partial.cs`
```
internal partial class GeneratedTypes
{
internal interface IUnionType
{
}

internal partial class A : IUnionType
{
}
}
```

Proposal: extend the current plugin config with an option called `partialTypes`.
Possible values by type:
* string: none (turns feature off), classes (marks all classes as partial), record (marks all record as partial), interfaces (marks all interfaces as partial), all (marks all classes, records and interfaces as partial).
* predicate function (typeName: string) => bool: decides for every type individually if it should be marked as partial
* set: a JavaScript set containing the types to mark as partial

### Your Example Website or App

https://codesandbox.io/p/devbox/github/mariusmuntean/graphql-code-generator-issue-sandbox-template-csharp-partial-types/tree/main/

### Steps to Reproduce the Bug or Issue

No repro needed, just an enhancement to allow more flexibility.

### Expected behavior

As a C# developer I expect to be able to extend the generated types with new functionality by using the C# partial class feature, but the generated classes aren't marked as partial.

### Screenshots or Videos

_No response_

### Platform

- OS: all
- `@graphql-codegen/c-sharp` version(s): 5.0.0

### Codegen Config File

_No response_

### Additional context

_No response_