Add switch to suppress auto-insertion of operationIndex [csharp-netcore]

Since version 6.0.0 an optional parameter operationIndex is being automatically added to all calls in csharp-netcore - and possibly other* - client side SDK code.

Eg

/// <summary>Get a list of lists.</summary>
/// <param name="listType">The type of list to return.</param>
/// <param name="operationIndex">Index associated with the operation.</param>
/// <returns>CollectionOfList</returns>
public CollectionOfList GetLists(string listType, int operationIndex = 0);

This behavior persists in version 6.0.1.

I can't find any reference to this in the changes for 6.0.0, or anywhere other a very brief comment in https://github.com/OpenAPITools/openapi-generator/pull/12090.

This a problem for us as it adds a parameter that is likely to confuse users of the third-party consumed .NET Standard 2.0 class library we generate from nearly 20 lengthy OpenAPI documents on a regular basis.

Assuming this behavior is intentional (ie, not a bug), we would like to see a configuration switch - eg, suppressOperationIndexes (default true) - to suppress the creation of these parameters. (Personally, I think it should really be addOperationIndexes (default false) but that's just an opinion based on previous version behavior.)

We have the option of using scripts to post-process the generated code but this is laborious to maintain and shouldn't really be necessary (IMHO).

* Tested version 6.0.1 with csharp and operationIndex is not added to the SDK.

Start by tracing where the csharp-netcore generator adds the operationIndex parameter, then review the behavior introduced around pull request 12090 and compare it with csharp generation. Done means a configuration switch can suppress the automatically generated parameter for csharp-netcore without affecting the existing opt-in behavior; verify the generated SDK output for both settings.