[REQ] [csharp] [generichost] Support query parameters with style: deepObject and explode: true

Is your feature request related to a problem? Please describe.

The OpenAPI spec allows you to define parameters that can be rendered as color[R]=100&color[G]=200&color[B]=150.

This can be written in the OpenApi yaml file as follows:

parameters:
  - name: color
    in: query
    style: deepObject
    explode: true
    schema:
      title: ColorParameter
      type: object
      properties:
        key:
          type: string
        value:
          type: string

This is related to the issue in #19893

Describe the solution you'd like

To implement this, we need to modify several files.

Dealing with explode

First, we need to fix the signature of parameters with 'explode: true'.
The spec states that when this is true:

Parameter values of type array or object generate separate parameters for each value of the array or key-value pair of the map.

For a query parameter with name: color, this would look like

/foo&color=blue&color=black&color=orange

Since the parameter can be used 0 or more times, we need to change the parameter to a collection type (like List<T>).
This can be done by modifying Operation.Signature.mustache to add a {{#isExplode}} ... {{/isExplode}} section.

Next, we have to modify the following to add all the values in the List<T> parameter to NameValueCollection
https://github.com/OpenAPITools/openapi-generator/blob/008c1a42ef39100b38a86fc67344ea44d4f32a8f/modules/openapi-generator/src/main/resources/csharp/libraries/generichost/api.mustache#L408-L443

Support deepObject style

This along with explode:true allows us to specify query parameters as someParam[field1]=value1&someParam[field2]=value2&someParam[field3]=value3

Since the parameter defines a schema with type: object and properties, the generator generates a model to represent this type. For deepObject styles, it only makes sense to define two properties: the field or column and the value. I think what we can do here is then generate a model that has

// from parameter.name
public string Name {get; set;}

// from parameter.schema.properties[0]
public string Column {get; set;}  // or Field

// from parameter.schema.properties[1]
public string Value {get; set;}

Then we can generate the query parameter as Name[Column]=Value

Building on top of the changes we made previously,

Describe alternatives you've considered

Additional context

Start with csharp/libraries/generichost/OperationSignature.mustache and the query-parameter handling in csharp/libraries/generichost/api.mustache around lines 408-443. Compare the current generated signatures and NameValueCollection handling with the OpenAPI deepObject and explode rules, and review related issue #19893. Done means generated Generic Host clients represent these parameters and produce the documented query forms.