[R] Add httr2 support (work in progress) (#13005)

* add httr2 support to r client gen

* fix headers

* add accepts, content-types

* update samples

* fix req

* update samples

* various fixes

* add data file test

* fix streaming, add tests

docs/generators/r.md

@@ -21,6 +21,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |errorObjectType|Error object type.| |null|
 |exceptionPackage|Specify the exception handling package|<dl><dt>**default**</dt><dd>Use stop() for raising exceptions.</dd><dt>**rlang**</dt><dd>Use rlang package for exceptions.</dd></dl>|default|
 |hideGenerationTimestamp|Hides the generation timestamp when files are generated.| |true|
+|library|HTTP library template (sub-template) to use|<dl><dt>**httr2**</dt><dd>httr2 (https://httr2.r-lib.org/)</dd><dt>**httr**</dt><dd>httr (https://cran.r-project.org/web/packages/httr/index.html)</dd></dl>|httr|
 |operationIdNaming|Naming convention for operationId (function name in the API)|<dl><dt>**PascalCase**</dt><dd>Pascal case (default)</dd><dt>**snake_case**</dt><dd>Snake case</dd><dt>**camelCase**</dt><dd>Camel case</dd></dl>|null|
 |packageName|R package name (convention: lowercase).| |openapi|
 |packageVersion|R package version.| |1.0.0|