feat: HTTP basic authentication secrets for network plugin (#367)

## Issue
N/A

## Description
Add support for creating basic authentication secrets for the network
plugin.

CA certificate secrets aren't required as they're either provided inline
in the network rule spec or via a secret reference. In `validatorctl` we
provide the certs inline in the spec, so we don't require a facility for
creating the secrets via Helm.

---------

Signed-off-by: Tyler Gillson <tyler.gillson@gmail.com>