Add common constants for HTTP status-code responses

[ayeshLK]

Description:
Provide common reusable constants for HTTP status-code responses.

Sample Usage
Developer should be able to use common constants for HTTP status-codes when developing a HTTP service.

service / on new http:Listener(9090) {
    resource function 'default hello(http:Request request) returns http:Ok|http:BadRequest {
        error? validationResponse = validate(request);
        if validationResponse is error {
            return http:BAD_REQUEST;
        }

        // implement logic
        return http:OK;
    }
}