Define a default memory limit for shutdown-manager

[ryanhristovski]

Description:
Currently the shutdown-manager sidecar does not have a default memory limit set, this can lead to uncontrolled memory usage, potentially causing node instability or OOMKilled errors. It's best practice to set the memory request equal to the memory limit to ensure consistent resource allocation and avoid unexpected scheduling issues.

I haven't yet benchmarked shutdown-managers memory usage to have a good idea of what to set the request/limit to if anyone has any suggestions.

[arkodg]

requests == limit sgtm, wdyt @envoyproxy/gateway-maintainers @envoyproxy/gateway-reviewers