feature request: `skip_if_present` for request/response headers to add

*Title*: `skip_if_present` for request/response headers to add

*Description*:
When using `request/response_headers_to_add`, the only options are either replace the header or append to it. Sometimes it may be preferable to simply ignore if the header is already set. This would add a boolean `skip_if_present` option to `core.HeaderValueOption` that defaults to `false`.