Support multi ssh-keys in git client

[Bibisha06]

What this PR does:

This PR implements support for multiple SSH keys in the Piped Git configuration. It introduces a new sshKeys list field to PipedGit while maintaining full backward compatibility with the existing single-key setup.

Key changes:

• Prioritized Config: Refactored AddSSHConfig to generate a composite ~/.ssh/config file that writes the legacy host entry first followed by entries in the sshKeys list, preserving SSH's first-match-wins logic.
• Secure File Handling: Identity files are created in a temporary directory with strict 0600 permissions via a new writeSSHKeyFile helper.
• Improved Lifecycle: The Git package now returns a temporary directory path to the Piped runtime, ensuring all generated identity files are removed in a single os.RemoveAll call.
• Validation: Added strict validation in pkg/configv1 to prevent duplicate host mappings and ensure configuration integrity.

Why we need it:

Some repositories may require distinct SSH keys (e.g., repository-specific deployment keys or keys across different organizations/domains) that cannot be handled by a single global SSH key.

Which issue(s) this PR fixes:

Fixes #4311

Does this PR introduce a user-facing change?:

• How are users affected by this change: Users can now optionally define multiple SSH keys in their Piped configuration. Existing configurations continue to work without modification.
• Is this breaking change: No.
• How to migrate (if breaking change): N/A.