[REQ] [typescript-fetch] Divide an API method into request and RequestOpts factory

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

I use openapi-generator with typescript-fetch for couple of months but I am missing one thing I had in dynamic swagger-js client.

Each operation in such client had urlify method.
You could pass all parameters there and it build a request url for you.

This is very useful for simple GET requests.
For example we have an API endpoint for fetching some images.
There are couple of path parameters there.
In swagger-js we could generate url on runtime using myOperation.urlify(params) (this is synchronous and doesn't call the server) and then pass such url into src of an <img /> tag.

This simplifies the implementation a lot.
I would like to have such functionality because I want to be immune to path structure changes.

Describe the solution you'd like

For each generated API Method we should generate and export additional synchronous Factory Function that takes the same parameters as the API Method and returns RequestOpts.

For example:
API Method:

async getProjectAvatar(requestParameters: GetProjectAvatarRequest): Promise<File>

Factory Function:

getProjectAvatarRequestOpts(requestParameters: GetProjectAvatarRequest): RequestOpts

So I could use it in my React code this way:

const avatarUrl = generatedClient.getProjectAvatarRequestOpts(someSpecificParamsForThisEndpoint);

return <img src={avatarUrl} />

Start at the typescript-fetch generator entry point and locate the templates or generation logic that emit API methods. Compare the generated method with the requested synchronous RequestOpts factory, ensuring it accepts the same request parameters and does not call the server. Done means generated clients expose the additional factory for each API method and produce a usable request URL.