Add additional_ca_cert_bundle input for DS v2 template

What does this MR do and why?

Add additional_ca_cert_bundle input for DS v2 template. The feature already works when passing the CI/CD variable but we want a corresponding input for consistency.

• add specs to verify CI/C variables backward compatibility
• fix typo in DS_ENABLE_VULNERABILITY_SCAN fallback logic

Changelog: added EE: true

References

#580462

Screenshots or screen recordings

Before	After

How to set up and validate locally

You need to have your GDK configured with HTTPS support.

1. configure the MY_CUSTOM_CA_CERT variable (e.g. in project's variables) with your custom CA root certificate. If you've used mkcert on MacOS that will be the content of the rootCA.pem file:

    cat ~/Library/Application\ Support/mkcert/rootCA.pem

2. create a project that includes the v2 DS template and speficies the input

   include:
     - template: Jobs/Dependency-Scanning.v2.gitlab-ci.yml
       inputs:
         additional_ca_cert_bundle: $MY_CUSTOM_CA_CERT

3. verify in the job log output that the DS analyzer can reach the sbom scan API of the instance

MR acceptance checklist

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