[Swift6] Rework query string encoding for all requests

[x-sheep]

#20997

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[4brunu]

Hi @x-sheep, thanks for creating this PR.
This PR is way bigger than I was expecting, I was expecting just a few minor changes.
Honestly I'm a bit concerned with the stability of Swift generator, because I found another bug #20997 (comment), and maybe there are still other bugs that we didn't find.
So my top priority now is stabilize the Swift generator.
With this in mind, I'm trying to avoid merge big PR, because big changes can create more instability.

[x-sheep]

Hi 4brunu,

This branch is very far from finished. I currently only have this PR open to experiment, and make use of the Bitrise CI.

I do think that calling encodeToQueryString on every parameter is a bad idea, and I want to replace this with another approach. So the amount of affected files is still going to be pretty large, since all generated functions will be changed.

When I'm ready for feedback, I'll give you a mention.