[CppRest] Support optional parameters

[fvarose]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
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Description of the PR

This PR adds support for optional parameters (those marked as required: false in a swagger spec) using boost::optional.
Users of the client code can now pass boost::none to API calls for any optional parameters they are not interested in using.

Notes:

• in: path params are not covered because they cannot be optional (see OpenAPI spec).
• in: body params are also not covered, as the implementation would be less trivial. The current behaviour for those params doesn't change, i.e. they are always required to make a call.
• This doesn't add any new linking dependencies as boost::optional is header-only, and boost itself is already required to compile the generated client code.

[fvarose]

cc @stkrwork @ravinikam

[wing328]

@fvarose thanks for the enhancemnet.

@ravinikam thanks for reviewing the change.