Created asmdef for sample + moved the sample to unity recommended directory (Samples~ instead of sample)

[kbobnis]

1. Before this change, when importing package, the package was fetching the sample directory as part of the package.
   Moving sample to special named directory Samples~ (https://docs.unity3d.com/Manual/cus-samples.html) will force the unity package manager UPM to skip this directory when importing package. An option to download sample is still visible in the UPM under RestClient package.
2. The sample uses RestClient package, therefore another asmdef was created for sample with dependency to RestClient.

Known problems:

1. Only when optionally downloading the sample: The sample can't find method: CleanDefaultHandlers - I don't know why
   Assets\Samples\RestClient for Unity\2.6.2\Sample\MainScript.cs(48,15): error CS0117: 'RestClient' does not contain a definition for 'CleanDefaultHeaders'

[kbobnis]

I have created an empty project with only this package imported to showcase this error: https://github.com/kbobnis/proyecto26-restclient-imported-with-sample

[jdnichollsc]

Oh about the error, that function was renamed to ClearDefaultHeaders 😅

@kbobnis Let me update that and thanks for your contribution mate! <3