teleport: Re-add possibility for overrides

[JuliusFreudenberger]

The interface for the overrides withRdpClient and extPatches was removed with #411095.

Resolves #411095 (comment).
Also has to be backported to restore the interface there as stated in #441311 (comment).

This is an intermediate fix to restore old functionality until #404946 or similar provides some other means.

Things done

• Built on platform:
  • x86_64-linux (using the override)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[JuliusFreudenberger]

@simonzkl, I tested a build with the old interface, which is restored with this PR. Could you please test, if the flag really comes into effect?

[JuliusFreudenberger]

Also pinging @philiptaron as you were also involved.

[nixpkgs-ci]

Backport failed for release-25.05, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-25.05
git worktree add -d .worktree/backport-444580-to-release-25.05 origin/release-25.05
cd .worktree/backport-444580-to-release-25.05
git switch --create backport-444580-to-release-25.05
git cherry-pick -x ec923b65ee39a1a7d599a903edab305784a5925b