[CppRest] Add Object and fix modelbase includes

[fvarose]

PR checklist

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• 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\.
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Description of the PR

This PR addresses two issues:

1. The Object.h header file does not exist, we replace it with ModelBase.h which provides the expected functionality (fix [CPPREST] Arbitrary object type results in a compilation error (missing Object.h) #6512). This was seemingly an early copy and paste mistake that wasn't caught by the PetStore example.
   Edit (14/11/2017): this PR provides a simple implementation of Object, wrapped around web::json::value from casablanca. This fixes [CPPREST] Arbitrary object type results in a compilation error (missing Object.h) #6512.
2. Fixed some relative include paths, to prevent clashes during linking when using two separate APIs that generate identical file names.

I've updated the sample code accordingly and managed to compile it without any problems.
This change has also been tested successfully on a codebase where two separate APIs coexist (which wasn't possible at all before).

[fvarose]

cc @stkrwork @ravinikam

[fvarose]

Update: I've had a second look and this actually doesn't solve the issue mentioned in #6512..
I will need to revise this by creating an actual Object class that derives from ModelBase and implements the virtual conversion functions.

[fvarose]

This PR is now ready to be reviewed again. It fixes #6512.
@wing328 @stkrwork @ravinikam

[wing328]

@fvarose thanks for the enhancement.

@stkrwork thanks for reviewing the change.

[murzic4]

hi all
Im not sure but it looks like types UUID and Number are missing as well

[wing328]

@murzic4 do you mind filing a PR to add those?