Add possibility to add custom configmap needed by optional init containers

[atouboulic]

Current Vector Version : vector 0.13.2

Proposal :

Add possibility to add optional custom configmap needed by optional init containers.

Use Case:

This would be usefull in order to setting up geoip VRL for example. The VRL requires access to the mmdb and currently, there is no way to set it up without creating a specific config map manually.

[spencergilbert]

This is probably possible today with extraVolumes, extraVolumeMounts, and initContainers but we can certainly improve the UX if that's the case.