Provide an option for merging of URL query string with params

[jhominal]

In light of the resolution of #3433, as I still believe that it is a common need to merge the query string from the URL with a params object, I propose implementing an option that would allow API users to specify the way that a query string in the URL will interact with a params dictionary.

• I would call that url_query_params_mode;
• I would like to implement 3 modes for this parameter:
  • replace: that is the behaviour from httpx >=0.28 - the query component from the URL is replaced wholesale;
  • merge: that is an approximation of the behaviour of httpx<0.28:
    • The query string from the URL is parsed, as a dictionary;
    • The parameters specified in client.params are then merged with the dictionary from the previous step
      • One of the peculiarities of merging is that, in the case where a parameter to merge has the same name as a parameter in the source, the incoming value will replace the previous one (as if merging dictionaries using |);
    • The parameters specified in request.params are then also merged with the dictionary from the previous step;
    • That final dictionary is then serialised to a query string, and set on the URL;
  • append: that is the behaviour that I personally find the most interesting, basically:
    • The query string from the URL is kept as is (no attempt is made to parse it);
    • The parameters specified in client.params are serialised to a query string, and appended to the URL;
    • The parameters specified in request.params are serialised to a query string, and appended to the URL:
    • I find that mode appealing for the following reasons:
      • This mode does not change any of the URL parameters that are already set on the URL string representation;
      • All parameters and all values specified at any level (client or request) will be found in the final URL;

What do you think? If anyone at @encode/maintainers thinks that idea is interesting, I would be happy to work on a PR.