[Phase 02] Support resource functions in OpenAPI Client Generation tool

[lnash94]

Description: Parent issue: https://github.com/ballerina-platform/ballerina-standard-library/issues/5366

During this 2nd phase, try to improve the client resource method body with the client resource method call in below way

string response = check httpClient->/some/endpoint(payload, headers, "application/json", name = "foo", id = 123);

Current Generated code

resource isolated function get api/v2/opportunities(string? embed = ()) returns OpportunityObject|error {
        string resourcePath = string `/api/v2/opportunities`;
        map<anydata> queryParam = {"embed": embed};
        resourcePath = resourcePath + check getPathForQueryParam(queryParam);
        OpportunityObject response = check self.clientEp->get(resourcePath);
        return response;
    }

With Improvement

resource isolated function get api/v2/opportunities(string? embed = ()) returns OpportunityObject|error {
        OpportunityObject response = check self.clientEp->/api/v2/opportunities/.get(embed);
        return response;
    }

[lnash94]

Once we are going to check the feasibility of this improvement in the client resource function we encounter this ,

• Given query parameters has parameter serialization details and the resource path should be included the details of the serialization. Example can be found here. For Stripe API there are scenarios where we have to send data structures like objects as query parameters.
  In such cases we need to serialize the parameter in one of the below ways

Other than query parameters, object can be send as a path parameter or header according to the OpenAPI 3.0 . Please find more information on related to parameter serialization in OpenAPI 3.0 here.

This scenario was handled previously by introducing a util function to generate the path according to the serialization.

resourcePath = resourcePath + check getPathForQueryParam(queryParam);
------------
# functions to generate query parameter path
isolated function  getPathForQueryParam(map<anydata> queryParam)  returns  string|error {
    string[] param = [];
    param[param.length()] = "?";
    foreach  var [key, value] in  queryParam.entries() {
        if  value  is  () {
            _ = queryParam.remove(key);
        } else {
            if  string:startsWith( key, "'") {
                 param[param.length()] = string:substring(key, 1, key.length());
            } else {
                param[param.length()] = key;
            }
            param[param.length()] = "=";
            if  value  is  string {
                string updateV =  check url:encode(value, "UTF-8");
                param[param.length()] = updateV;
            } else {
                param[param.length()] = value.toString();
            }
            param[param.length()] = "&";
        }
    }
    _ = param.remove(param.length()-1);
    if  param.length() ==  1 {
        _ = param.remove(0);
    }
    string restOfPath = string:'join("", ...param);
    return restOfPath;
}

but given the improvement resource method

OpportunityObject response = check self.clientEp->/api/v2/opportunities.get(embed);

we can't handle the parameter serialization. Therefore we have to move with the current implementation [1] until we receive some support from http to handle parameter serialization-related issues https://github.com/ballerina-platform/ballerina-standard-library/issues/1747 [1]

resource isolated function get api/v2/opportunities(string? embed = ()) returns OpportunityObject|error {
        string resourcePath = string `/api/v2/opportunities`;
        map<anydata> queryParam = {"embed": embed};
        resourcePath = resourcePath + check getPathForQueryParam(queryParam);
        OpportunityObject response = check self.clientEp->get(resourcePath);
        return response;
    }