[YAML parser] Support command line yaml parameter string

[anup-pem]

#235 takes of supporting parsing YAML parameter file.

The above work needs to be extended to support command line string yaml parameters. The API that needs to be implemented for this functionality is

/// \brief Parse the YAML string, initialize and populate params_st
/// \param[in] yaml_string is the YAML parameter string to be parsed
/// \param[in/out] params_st points to the populated paramter struct
/// \return true on success and false on failure
RCL_YAML_PARAM_PARSER_PUBLIC
bool rcl_parse_yaml_file(
  const char * yaml_string,
  rcl_params_t * params_st);