[java-jersey2] Conditionally include http signature mustache template

Created by: sebastien-rosset

The HTTP signature auth mustache template should be included only when the OAS document has the http-signature security scheme. I've seen some builds failing, I'm not sure how the merge to master was possible.

https://circleci.com/gh/OpenAPITools/openapi-generator/16032?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

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