Load existing certs from pkcs12 store

[c0d1ngm0nk3y]

Summary

When writing a pkcs12 key store, we read first the existing certificates like done in the jks case. The implementation did remove any existing certs before.

Upport of https://github.com/paketo-buildpacks/libjvm/pull/344

Use Cases

The keystore provided by the JRE already contains some certificates.

Checklist

• [X] I have viewed, signed, and submitted the Contributor License Agreement.
• [X] I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [ ] I have added an integration test, if necessary.
• [X] I have reviewed the styleguide for guidance on my code quality.
• [X] I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[phil9909]

@dmikusa can you merge this as well? Same PR as #344, but targeting the release-2.x branch instead of main.