Summary: Add an optional action input that allows providing a custom cacerts file to be used by the installed JDK.

Background: PR #284 proposed this capability, but the branch is now very old and heavily conflicted. The feature request can continue in a fresh implementation.

Proposed behavior:

• New optional input for path to a cacerts file
• If provided, configure the installed Java runtime to use that cacerts content
• Keep default behavior unchanged when input is not set

Scope:

• Re-implement on current main instead of reusing the old branch
• Add tests for enabled and default paths
• Document usage and caveats for hosted and self-hosted runners

Out of scope:

• Unrelated runtime or lockfile changes

Reference: PR #284