[Feature] Allow setting pod sysctls

[maxenglander]

Description

Allow setting sysctls on pods created by the operator, using the mechanism Kubernetes exposes (pod security context) for doing so.

Use case

Organizations using Vitess may have a database team that operates Vitess on top of Kubernetes clusters managed by an infrastructure team. The infrastructure team may set sysctl defaults which are recommended for the general case, but not ideal for Vitess.

The infra team may be unwilling to change the global defaults, but be willing to let the database team override namespace-scoped sysctls in Vitess-managed pods. In that case, the database team will need some way to set namespace-scoped sysctls.

Thanks

Thanks to @bluecrabs007 for bringing this up w/ me.