Introduce API to rewrite samples

[vmishenev]

#3108 Finally I have decided to use SampleRewriter as an extension point. In case of using it as an argument of resolveSample:

• I am not sure SampleTransformer will not changed in the future. It also changes pages resources. Maybe it even makes sense to get rid of it or transform it to RunnableSampleTransformer
• With overriding of some SampleTransformer 's functions we can have the same situation like with HtmlRender
• Using an extension point is more obvious for users to rewrite samples