Add support for Boolean in R client generator

Created by: sneumann

Hi @Ramanth @saigiridhar21 et al,

as mentioned in the branch name, this PR is in response to (my) #9468 . Unfortunately I am not a openapi-generator hacker, so I have no idea if this is what's required, and its a best-effort approach. What I did:

1. extend the model.mustache in two places to check for the correct class, and I did that in analogy to the lines before/after my insertion
2. extend the model.mustache in two places further down, where numbers are printed with %d% and everything else was printed as "%s". The latter needed changing. I HOPE what I did results in unchanged "%s" for everything but Boolean, and %s for booleans
3. finally the logical values are needed in lowercase, so I surrounded self${{baseName}}bytolower()` in case of booleans.

The second commit adds the petstore, changed only where the logical complete is involved. And apparently my editor removed an empty line at the end of the template, so some changes remove only whitespace I do not know exactly how else I can test everything. Hope that helped nevertheless, Yours, Steffen

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