Expand Resource and PagedResources generics for Java Rest

[bzylstra]

• add new hook to codegen config: postProcessFileContents that is invoked after the file contents are generated from the template but before they are written to the file. Default is to leave contents as found
• for java client code generator, if the library is resttemplate and template is for the api, expand all occurrences of PagedResourcesBlah into PagedResources<Blah> and ResourceBlah into Resource<Blah>

US203263

PR checklist

• [X] Read the contribution guidelines.
• [X] 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\.
• [X] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language. I am not sure what it means to 'copy the technical committee'. Here is the list of from the indicated website for Java. @bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09)

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc) Close #7081 Add a step in the code generation after the templates are applied that turns references such as PagedResourcesResourceMyStuff back into PagedResources<Resource<MyStuff>> Applied only to Java codegens using the resttemplate library.