Add optionnal initContainer to fix sysctl config

[AlexisDucastel]

This request will add an optionnal flag to helm chart values.yaml, that let people add an initContainer to fix sysctl missing "net.ipv4.ip_forward=1" issue. (like #140)

Here is an exrtract of the documentation I added to values/yaml :

sysctlInitContainer flag adds an initContainer named "init-sysctl" to wg-access-server deployment.
The goal is to set the sysctl net.ipv4.ip_forward=1 to allow packet routing through node.
This initContainer needs to run as privileged, but this is only limited to the initContainer run time, the main container will remain unprivileged as expected.
Use case :
DNS is functionning properly through VPN but does not work for standard traffic.

NB : If you have no problem with wireguard traffic, you should not enable this initContainer

[SoarinFerret]

I'd love to see this merged - has bit me more than once at this point. I went ahead and published my own chart with these changes for internal use, but I'd prefer to use the upstream one long term.