fix: Replace placeholder for initContainers in broker pods

[diconico07]

What this PR does / why we need it:
Broker pods can have init containers that need access to akri resources.

This commit adds the logic to replace the {{PLACEHOLDER}} for init containers in the same way it is done for regular containers.

Fix #645
Special notes for your reviewer:

If applicable:

• this PR has an associated PR with documentation in akri-docs
• this PR contains unit tests
• added code adheres to standard Rust formatting (cargo fmt)
• code builds properly (cargo build)
• code is free of common mistakes (cargo clippy)
• all Akri tests succeed (cargo test)
• inline documentation builds (cargo doc)
• all commits pass the DCO bot check by being signed off -- see the failing DCO check for instructions on how to retroactively sign commits

[diconico07]

/version patch