[typescript-fetch] Allow to generate client without runtime checks

Created by: acote-coveo

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I tried to use the typescript-fetch client with a spec that use union and saw that the types were good but the serialization/deserialization wasn't working pretty well.

The main problems are:

• Lack of information about oneOf (it only contains a list of data types. A datatype can be OneType or Array<Type1 | Type2 | string | boolean>)
• Difficulty to implement imports due to the nested levels of typing.

Example of what was badly generated

Below, we have a list of oneOf data types to map. Thus, to serialize them, we call dataType[ToJson]. We then end up with something like boolean | AdvancedGenerateQueryFieldParameter ToJson

export function ToIncludeParametersToJSON(value?: ToIncludeParameters | null): any {
    if (value === undefined) {
        return undefined;
    }
    if (value === null) {
        return null;
    }
    return {
        
        'fields': value.fields === undefined ? undefined : ((value.fields as Array<any>).map(string | AdvancedQueryFieldParametersToJSON)),
        'summary': boolean | AdvancedGeneratedQueryFieldParameterToJSON(value.summary),
        'excerpt': boolean | AdvancedGeneratedQueryFieldParameterToJSON(value.excerpt),
        'highlighting': HighlightingSettingsToJSON(value.highlighting),
    };
}

Simple solution

A bit like what has been done with the Axios client,I added a flag that allows to generate a client without any runtime checks.

So we simplify the client to only generate the interfaces and cast the input/output to these. For a small effort, we now are able to have a simple client that compiles.

On a subsequent pull request, I would like to address the problem of unions by also providing serialization/deserialization (runtime checks).