Support for multiple values of query parameters

For a request containing multiple values of a parameter, e.g. `/api?q=val1&q=val2&q=val3`,
and the configuration `params.path = "/path?q={param.q}"` all the values should be added to the target url.
Only one value is currently added, because `RequestPlaceholderSubstitutor` calls`request.getParams().get()` instead of `request.getParams().getAll()`. That also would require change of interface `PlaceholderSubstitutor` to return multiple values.
Please consider support for multiple values of http headers too.

Start with RequestPlaceholderSubstitutor and the PlaceholderSubstitutor interface, comparing the current single-value parameter handling with the requested multiple-value behavior. Verify that repeated query parameters are all represented in the target URL, then assess the stated extension to multiple HTTP header values; the issue names no tests or files beyond these entry points.

Written by the indexing model from the issue text.