[cpp-qt] Add global server index setter for all operations

[MartinDelille]

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Ping @ravinikam @stkrwork @etherealjoy @muttleyxd @aminya

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Summary by cubic

Add a global server index setter to the C++ Qt client so you can switch the server for all operations at once. Updates templates and docs, and regenerates samples.

• New Features
  • Generator: add setServerIndex(int) to apply a server index across all operations via _serverIndices iteration.
  • Docs/Samples: document the new method in README.mustache and update petstore cpp-qt samples.

^{Written for commit 4fb3dd8. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 25 files

[wing328]

thanks for the PR

what about adding a docstring similar to the comment block above (line 59 - 62) ?

[wing328]

i'll do it in another PR instead. thanks for the contribution.