[gradle] Add recommendations to validate task

[jimschubert]

Adds recommendation/validation to the validate task in the gradle plugin.
This fixes #335 although there's no addition to the maven plugin, because none of CLI/Gradle/Maven do custom recommendations or validation in the generate task.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[jimschubert]

cc @OpenAPITools/generator-core-team

[jimschubert]

Caches on CI appear to have gotten messed up with this PR. I've opened another to retry.