CSharp plugin argument classes not created

**Describe the bug**
There are not created argument classes when using c sharp plugin

**To Reproduce**
Extremely simple. Just compare homepage of this project (default is typescript in yml file ) to this for example

overwrite: true
schema: schema.graphql
generates:
RimacCmvdsClient.Models/Models.cs:
plugins:
- c-sharp
config:
namespaceName: e
scalars:
DateTime: DateTime

QuerySearchArgs is missing !

Sandbox has timeouts but reproducing this is trivial. Just on home page of this site use c-sharp plugin

1. My GraphQL schema:

```graphql
scalar Date

schema {
query: Query
}

type Query {
me: User!
user(id: ID!): User
allUsers: [User]
search(term: String!): [SearchResult!]!
myChats: [Chat!]!
}

enum Role {
USER,
ADMIN,
}

interface Node {
id: ID!
}

union SearchResult = User | Chat | ChatMessage

type User implements Node {
id: ID!
username: String!
email: String!
role: Role!
}

type Chat implements Node {
id: ID!
users: [User!]!
messages: [ChatMessage!]!
}

type ChatMessage implements Node {
id: ID!
content: String!
time: Date!
user: User!
}

```

2. My GraphQL operations:

```graphql
# Put your operations here
```

3. My `codegen.yml` config file:

```yml
overwrite: true
schema: schema.graphql
generates:
RimacCmvdsClient.Models/Models.cs:
plugins:
- c-sharp
config:
namespaceName: e
scalars:
DateTime: DateTime
```

**Expected behavior**
Created argument classes for arguments like in typescript plugin (classes with Args sufix)

**Environment:**

- OS:
- `@graphql-codegen/...`:
- NodeJS: 14.15.4

**Additional context**