Provide a convent API for form-urlencoded

[manuranga]

Currently, we have to call url:encode multiple times to do form-urlencoded. Please provide a convent api to do following.

string payload = "From=" + check url:encode(FROM_NO, "utf-8") +
                 "&To=" + check url:encode(req.phoneNo, "utf-8") +
                 "&Body=" + check url:encode(body, "utf-8");

[manuranga]

Came up during https://github.com/ballerina-guides/enterprise-integration-patterns/pull/75/files#r1326856694 We need to fix the pattern after this is implemented.