Apply a percentage when calculating memory limit on control plane deployment

[jijiechen]

Description

We've implemented GOMEMLIMIT based on resources.memory.limit on CP deployoment by this PR #10879

It's suggested to leave some headroom to make the GC run more frequently so that we get a smaller chance of being OOM-killed.

We may use 15% headroom for our case since the memory can grow quickly sometimes when new DPs are rolling out quickly. GOMEMLIMIT is not a hard limit to the process, it'll only impact the behaviour of the GC.

In this case, a good rule of thumb is to leave an additional 5-10% of headroom to account for memory sources the Go runtime is unaware of.
Ref: https://tip.golang.org/doc/gc-guide#Suggested_uses

We may use a helm helper like this to covert user provided values dynamitically:
kubernetes/kubernetes#91514 (comment)

Other charts are doing similar handling:
https://github.com/bitnami/charts/blob/main/bitnami/scylladb/templates/_helpers.tpl#L224