[R] improve binary/text response handling

[mattpollock]

@Ramanth, @saigiridhar21, @wing328

This PR fixes a bug when handling an endpoint that returns binary data (e.g. application/gzip). It is really a follow-up to #17626.

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[wing328]

shall we keep the snake case convention for the time being?

changing it results in breaking changes (without fallback) which is not something we want to do in the next minor release v7.11.0 (FYI we allow breaking changes with fallback in minor release)

[wing328]

to use PascalCase, I can think of a solution using lambda but I prefer we deal with it in another PR instead.

[mattpollock]

Sure, I'll revert to snake case. I switched to pearl based on the contributing guide, but I appreciate avoiding breaking changes.

[mattpollock]

done

[wing328]

what about adding these functions to api_client (which contains other utility functions)?

otherwise we will be repeating the same set of functions (private) in every auto-generated API files

[mattpollock]

Done. Note that the code will be appear once in generated clients, but will need to be maintained in 2 places:

1. modules/openapi-generator/src/main/resources/r/api_client.mustache
2. modules/openapi-generator/src/main/resources/r/libraries/httr2/api_client.mustache

[wing328]

thanks. we can use partial mustache files but that can be done in a separate PR instead.

[wing328]

@mattpollock thanks for the PR. I left some feedback. Definitely want to get this merged to improve the R client generator.

Feel free to PM me via Slack to further discuss this PR or if you need any help: https://join.slack.com/t/openapi-generator/shared_invite/zt-2uoef5v0g-XGwo8~2oJ3EoziDSO1CmdQ

[wing328]

if there's no further feedback from anyone. we will merge this PR over the weekend.

thanks again for the contribution.